feat: ship the skill set as a native Claude Code plugin

Add a Claude Code plugin marketplace so the promoted skills can be
installed as a managed, read-only bundle (subscribe rather than fork),
alongside the existing skills.sh installer.

- Enrich .claude-plugin/plugin.json with marketplace metadata (version,
  description, author, license, keywords); keep the curated promoted-only
  skills array.
- Add .claude-plugin/marketplace.json making the repo its own
  single-plugin marketplace (mattpocock-skills@mattpocock).
- README: document /plugin install alongside skills.sh, and the
  subscribe-vs-fork tradeoff.
- CLAUDE.md: extend the promoted-set invariant to cover marketplace.json
  and plugin.json/package.json version sync.
- ADR 0002: record why Claude ships now and a native Codex plugin is
  deferred (Codex skills field is single-path + drops symlinks, which
  can't express a curated subset of a bucketed repo without a restructure).

Verified: 'claude plugin validate . --strict' passes, and marketplace
add -> install resolves all 21 promoted skills at v1.2.0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.agents/adr/0002-ship-as-a-claude-code-plugin.md

@@ -0,0 +1,28 @@
+# Ship the skill set as a native Claude Code plugin; defer a native Codex plugin
+
+These skills have always been installable via [skills.sh](https://skills.sh/mattpocock/skills) (`npx skills add mattpocock/skills`), which copies editable skill files into a user's project across Claude Code, Codex, and other Agent-Skills-standard harnesses. A recurring request is a **plug-and-play** distribution: subscribe to the set as a read-only, always-current bundle you don't edit, rather than a fork you own. That is exactly what native plugin systems provide.
+
+We ship a native **Claude Code plugin** and, for now, **defer** a native **Codex plugin**. The split is forced by how each ecosystem's plugin manifest selects skills, against this repo's bucketed layout.
+
+## The constraint: bucketed skills vs. single-path selection
+
+Skills live in bucket folders under `skills/` — `engineering/` and `productivity/` are **promoted** (shipped); `misc/`, `personal/`, `in-progress/`, and `deprecated/` are **not**. A plugin must expose only the promoted set, which spans two of those bucket folders.
+
+- **Claude Code** — `.claude-plugin/plugin.json` accepts `skills` as an **array of explicit skill-directory paths**. We list the promoted skills one by one, exclude everything else with zero ambiguity, and add `.claude-plugin/marketplace.json` so the repo is its own single-plugin marketplace. Verified end to end: `claude plugin validate . --strict` passes, and `marketplace add` → `install` resolves all promoted skills.
+
+- **Codex** — `.codex-plugin/plugin.json` accepts `skills` only as a **single path string** (arrays are rejected with `missing or invalid plugin.json`), and Codex discovers `SKILL.md` files recursively under it. There is no way to name two bucket folders, or to curate a subset, from one path. Two escape hatches were tested and rejected:
+  - Pointing at `./skills/` would also ship `deprecated/`, `in-progress/`, `personal/`, and `misc/` — retired, draft, and personal skills we deliberately don't promote.
+  - A curated flat directory of **symlinks** into the buckets does not survive install: Codex copies the plugin tree into its cache and **drops symlinks**, so the skills arrive empty.
+
+The only robust ways to give Codex a single promoted-only path are (a) **restructure** so `skills/` contains only promoted skills (moving the non-promoted buckets out — a large blast radius across `CLAUDE.md`, `scripts/link-skills.sh`, the bucket READMEs, and the local dev workflow that relies on `in-progress/` and `personal/`), or (b) **commit duplicate copies** of promoted skills into a flat directory (a sync burden and a second source of truth). Both are structural decisions, not something to bundle into shipping the Claude plugin. This is very likely the original, half-remembered reason a plugin wasn't shipped earlier: the manifest formats didn't cleanly express a curated subset of a bucketed repo.
+
+## Decision
+
+- Ship the **Claude Code plugin** now (`.claude-plugin/plugin.json` + `.claude-plugin/marketplace.json`), curated to the promoted set, as the headline v1.2 deliverable.
+- Keep **skills.sh** as the universal installer — it already serves Codex and other harnesses today, so no Codex user is left without an install path.
+- **Defer** the native Codex plugin until we decide between restructuring `skills/` to promoted-only vs. committing a generated flat copy. Revisit when Codex either supports a `skills` array / include-list or preserves symlinks on install.
+
+## Invariants this creates
+
+- Every promoted skill has an entry in `.claude-plugin/plugin.json`'s `skills` array (this already stood as a `CLAUDE.md` rule; it now also gates the plugin's contents).
+- `.claude-plugin/plugin.json`'s `version` tracks `package.json`'s version — bump both together on release. Claude uses the plugin `version` to decide when installed users see an update.

.changeset/prototype-primary-source.md

@@ -0,0 +1,5 @@
+---
+"mattpocock-skills": minor
+---
+
+Reword how the **`prototype`** skill handles its artifacts around a single idea: **the prototype is a primary source**. Rather than being deleted once it's answered its question, the prototype is captured as runnable evidence on a throwaway branch (`prototype/<name>`) out of main, with a context pointer to it left on the implementation issue — so the main branch keeps only the validated decision while the exploration stays findable. The answer (verdict + question) is still captured durably in an issue/ADR/commit.

.changeset/ship-as-claude-plugin.md

@@ -0,0 +1,12 @@
+---
+"mattpocock-skills": minor
+---
+
+Ship the skill set as a native **Claude Code plugin**. The repo is now its own single-plugin marketplace, so you can subscribe to the promoted skills as a managed, read-only bundle instead of copying editable files:
+
+```
+/plugin marketplace add mattpocock/skills
+/plugin install mattpocock-skills@mattpocock
+```
+
+`.claude-plugin/plugin.json` gains full marketplace metadata (version, description, author, license, keywords) and a sibling `.claude-plugin/marketplace.json` lists the plugin. `skills.sh` remains the universal installer (and the path for Codex and other harnesses today); a native Codex plugin is deferred — see `.agents/adr/0002-ship-as-a-claude-code-plugin.md` for why.

.claude-plugin/marketplace.json

@@ -0,0 +1,23 @@
+{
+  "name": "mattpocock",
+  "owner": {
+    "name": "Matt Pocock",
+    "url": "https://www.aihero.dev"
+  },
+  "description": "Matt Pocock's skills for real engineering, as an installable Claude Code plugin.",
+  "plugins": [
+    {
+      "name": "mattpocock-skills",
+      "source": "./",
+      "description": "Matt Pocock's agent skills for real engineering — grilling, spec/ticket flows, TDD, code review, domain modelling and more.",
+      "category": "engineering",
+      "keywords": [
+        "engineering",
+        "skills",
+        "tdd",
+        "code-review",
+        "grilling"
+      ]
+    }
+  ]
+}

.claude-plugin/plugin.json

@@ -1,5 +1,23 @@
 {
   "name": "mattpocock-skills",
+  "version": "1.2.0",
+  "description": "Matt Pocock's agent skills for real engineering — grilling, spec/ticket flows, TDD, code review, domain modelling and more. Plug-and-play, not vibe coding.",
+  "author": {
+    "name": "Matt Pocock",
+    "url": "https://www.aihero.dev"
+  },
+  "homepage": "https://www.aihero.dev/s/skills-newsletter",
+  "repository": "https://github.com/mattpocock/skills",
+  "license": "MIT",
+  "keywords": [
+    "engineering",
+    "skills",
+    "tdd",
+    "code-review",
+    "grilling",
+    "domain-modeling",
+    "productivity"
+  ],
   "skills": [
     "./skills/engineering/ask-matt",
     "./skills/engineering/diagnosing-bugs",

CLAUDE.md

@@ -7,7 +7,9 @@ Skills are organized into bucket folders under `skills/`:
 - `in-progress/` — drafts not yet ready to ship
 - `deprecated/` — no longer used
 
-Every skill in `engineering/` or `productivity/` (the **promoted** buckets) must have a reference in the top-level `README.md` and an entry in `.claude-plugin/plugin.json`. Skills in `misc/`, `personal/`, `in-progress/`, and `deprecated/` must not appear in either.
+Every skill in `engineering/` or `productivity/` (the **promoted** buckets) must have a reference in the top-level `README.md` and an entry in `.claude-plugin/plugin.json`'s `skills` array (the Claude Code plugin ships exactly the promoted set). Skills in `misc/`, `personal/`, `in-progress/`, and `deprecated/` must not appear in either.
+
+The repo is also its own single-plugin Claude Code marketplace: `.claude-plugin/marketplace.json` lists the one `mattpocock-skills` plugin. When bumping the release version, keep `.claude-plugin/plugin.json`'s `version` in sync with `package.json`'s — Claude uses the plugin `version` to decide when installed users see an update. Run `claude plugin validate . --strict` after touching either manifest. Why a Claude plugin but not (yet) a Codex one lives in [.agents/adr/0002-ship-as-a-claude-code-plugin.md](./.agents/adr/0002-ship-as-a-claude-code-plugin.md).
 
 Each skill entry in the top-level `README.md` must link the skill name to its `SKILL.md`.
 

README.md

@@ -39,6 +39,33 @@ npx skills@latest add mattpocock/skills
 
 4. Bam - you're ready to go.
 
+## Install as a Claude Code plugin
+
+Prefer a plug-and-play install you don't maintain by hand? These skills also ship as a native [Claude Code plugin](https://code.claude.com/docs/en/plugins). Instead of copying editable files into your repo, the plugin installs the whole skill set as a managed bundle that updates when I ship a new version — you subscribe rather than fork.
+
+Inside Claude Code:
+
+```
+/plugin marketplace add mattpocock/skills
+/plugin install mattpocock-skills@mattpocock
+```
+
+Or from your shell:
+
+```bash
+claude plugin marketplace add mattpocock/skills
+claude plugin install mattpocock-skills@mattpocock
+```
+
+Then run `/setup-matt-pocock-skills` once per repo, exactly as in the quickstart above.
+
+Two ways to install, two philosophies:
+
+- **[skills.sh](https://skills.sh/mattpocock/skills)** copies the skills into your project so you can hack on them and make them your own.
+- **The plugin** keeps them as a read-only, always-current bundle you don't edit — best when you just want my set to work and follow along as it evolves.
+
+> Using Codex or another agent? The [skills.sh installer](https://skills.sh/mattpocock/skills) already installs these skills into Codex and other Agent-Skills-standard harnesses today. A native Codex plugin is on the roadmap — see [`.agents/adr/0002-ship-as-a-claude-code-plugin.md`](./.agents/adr/0002-ship-as-a-claude-code-plugin.md).
+
 ## Why These Skills Exist
 
 I built these skills as a way to fix common failure modes I see with Claude Code, Codex, and other coding agents.

docs/engineering/prototype.md

@@ -31,9 +31,11 @@ The question decides the shape, and there are two shapes:
 
 Picking the wrong branch wastes the whole prototype, so the question comes first. Both branches keep state in memory, run from one command, and surface the full state on every step.
 
-## The answer is the artifact
+## Keep the prototype as a primary source
 
-The code is disposable; the **answer** is the only thing worth keeping. When the prototype has settled its question, capture the verdict somewhere durable — a commit message, an ADR, an issue, or a `NOTES.md` next to it — alongside the question it answered, then delete or absorb the code. A prototype left rotting in the repo has outlived its purpose.
+A finished prototype leaves two things. The **answer** — the verdict plus the question it settled — is what you capture durably (a commit message, an ADR, an issue). The **prototype itself is a primary source** — the runnable evidence the answer came from.
+
+The prototype doesn't belong in the main branch: no tests, no error handling, nothing to maintain. But that's not a reason to destroy it. Once the answer is captured, fold any validated decision into the real code, then capture the prototype on a throwaway branch — out of main, never merged — and leave a context pointer to it on the implementation issue. The main branch stays clean; the raw exploration stays one click away for anyone who wants to re-run it. A prototype left rotting in the main branch has outlived its purpose — a prototype captured as a primary source on a side branch hasn't.
 
 ## Where it fits
 

docs/engineering/to-spec.md

@@ -57,4 +57,3 @@ grill-with-docs → to-spec → to-tickets → implement → code-review
 ```
 
 Reach for it after the plan and domain language are resolved, and before you break the work into implementation tickets. Its key neighbours are [grill-with-docs](https://aihero.dev/skills-grill-with-docs), which sharpens the context so the spec is precise, and [to-tickets](https://aihero.dev/skills-to-tickets), which turns the spec into a set of tickets for [implement](https://aihero.dev/skills-implement) to build. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.
-</content>

docs/engineering/to-tickets.md

@@ -54,4 +54,3 @@ grill-with-docs → to-spec → to-tickets → implement → code-review
 ```
 
 It sits between [to-spec](https://aihero.dev/skills-to-spec), which hands it a settled spec with user stories to slice against, and [implement](https://aihero.dev/skills-implement), which builds each ticket, driving [tdd](https://aihero.dev/skills-tdd) internally to write the tests test-first, before its [code-review](https://aihero.dev/skills-code-review) pass. Work the frontier one ticket per fresh context, clearing between them. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.
-</content>

skills/engineering/prototype/LOGIC.md

@@ -36,7 +36,7 @@ The right shape depends on the question:
 
 Pick whichever shape best fits the question being asked, *not* whichever is easiest to wire to a TUI. Keep it pure: no I/O, no terminal code, no `console.log` for control flow. The TUI imports it and calls into it; nothing flows the other direction.
 
-This is what makes the prototype useful past its own lifetime. When the question's been answered, the validated reducer / machine / function set can be lifted into the real module — the TUI shell gets deleted.
+This is what makes the prototype useful past its own lifetime: when the question's been answered, the validated reducer / machine / function set can be lifted into the real module on its own.
 
 ### 4. Build the smallest TUI that exposes the state
 
@@ -66,9 +66,9 @@ If the host project has no task runner, just put the command at the top of the p
 
 Give the user the run command. They'll drive it themselves; the interesting moments are when they say "wait, that shouldn't be possible" or "huh, I assumed X would be different" — those are the bugs in the _idea_, which is the whole point. If they want new actions added, add them. Prototypes evolve.
 
-### 7. Capture the answer
+### 7. Capture the answer and the prototype
 
-When the prototype has done its job, the answer to the question is the only thing worth keeping. If the user is around, ask what it taught them. If not, leave a `NOTES.md` next to the prototype so the answer can be filled in (or filled in by you, if you've watched the session) before the prototype gets deleted.
+Once the prototype has answered its question, capture the answer, then capture the prototype the way the [SKILL](SKILL.md) describes. The logic-specific mapping: the validated reducer / machine / function set lifts into the real module (the decision, absorbed); the TUI shell rides along to the throwaway branch that keeps the prototype as a primary source.
 
 ## Anti-patterns
 

skills/engineering/prototype/SKILL.md

@@ -21,10 +21,6 @@ The two branches produce very different artifacts — getting this wrong wastes
 1. **Throwaway from day one, and clearly marked as such.** Locate the prototype code close to where it will actually be used (next to the module or page it's prototyping for) so context is obvious — but name it so a casual reader can see it's a prototype, not production. For throwaway UI routes, obey whatever routing convention the project already uses; don't invent a new top-level structure.
 2. **One command to run.** Whatever the project's existing task runner supports — `pnpm <name>`, `python <path>`, `bun <path>`, etc. The user must be able to start it without thinking.
 3. **No persistence by default.** State lives in memory. Persistence is the thing the prototype is _checking_, not something it should depend on. If the question explicitly involves a database, hit a scratch DB or a local file with a clear "PROTOTYPE — wipe me" name.
-4. **Skip the polish.** No tests, no error handling beyond what makes the prototype _runnable_, no abstractions. The point is to learn something fast and then delete it.
+4. **Skip the polish.** No tests, no error handling beyond what makes the prototype _runnable_, no abstractions. The point is to learn something fast.
 5. **Surface the state.** After every action (logic) or on every variant switch (UI), print or render the full relevant state so the user can see what changed.
-6. **Delete or absorb when done.** When the prototype has answered its question, either delete it or fold the validated decision into the real code — don't leave it rotting in the repo.
-
-## When done
-
-The _answer_ is the only thing worth keeping from a prototype. Capture it somewhere durable (commit message, ADR, issue, or a `NOTES.md` next to the prototype) along with the question it was answering. If the user is around, that capture is a quick conversation; if not, leave the placeholder so they (or you, on the next pass) can fill in the verdict before deleting the prototype.
+6. **Capture it when done.** Fold any validated decision into the real code, then capture the prototype itself as a **primary source**: commit it to a throwaway branch, out of main, and leave a context pointer to that branch on the implementation issue. Capture the answer too — the verdict and the question it settled — in the issue or a commit. The main branch keeps only the validated decision.

skills/engineering/prototype/UI.md

@@ -97,12 +97,12 @@ Surface the URL (and the `?variant=` keys). The user will flip through whenever
 
 ### 6. Capture the answer and clean up
 
-Once a variant has won, write down which one and why (commit message, ADR, issue, or a `NOTES.md` next to the prototype if running AFK and the user hasn't responded yet). Then:
+Once a variant has won, capture the answer — which variant and why — then capture the prototype the way the [SKILL](SKILL.md) describes. Fold the winner into the real code and move the rest onto the throwaway branch, not into main:
 
-- **Sub-shape A** — delete the losing variants and the switcher; fold the winner into the existing page.
-- **Sub-shape B** — promote the winning variant to a real route, delete the throwaway route and the switcher.
+- **Sub-shape A** — fold the winner into the existing page; drop the losing variants and the switcher from main.
+- **Sub-shape B** — promote the winning variant to a real route; drop the throwaway route and the switcher from main.
 
-Don't leave variant components or the switcher lying around. They rot fast and confuse the next reader.
+The full set of variants is the primary source, so it lands on the throwaway branch, not the bin — variant components and the switcher left in the main branch rot fast and confuse the next reader.
 
 ## Anti-patterns
 

skills/engineering/to-tickets/SKILL.md

@@ -105,4 +105,3 @@ The end-to-end behaviour this ticket makes work, from the user's perspective —
 In either form, avoid specific file paths or code snippets — they go stale fast. Exception: if a prototype produced a snippet that encodes a decision more precisely than prose can (state machine, reducer, schema, type shape), inline it and note briefly that it came from a prototype. Trim to the decision-rich parts — not a working demo, just the important bits.
 
 Work the frontier one ticket at a time with `/implement`, clearing context between tickets.
-</content>

skills/in-progress/README.md

@@ -8,3 +8,4 @@ Skills that are still being developed. They're not ready to ship — expect roug
 - **[writing-fragments](./writing-fragments/SKILL.md)** — Grilling session that mines you for fragments — heterogeneous nuggets of writing — and appends them to a single document as raw material for a future article.
 - **[writing-shape](./writing-shape/SKILL.md)** — Take a markdown file of raw material and shape it into an article paragraph by paragraph, arguing format choices at each step.
 - **[claude-handoff](./claude-handoff/SKILL.md)** — Hand the current conversation off to a fresh background agent that picks up the work immediately, seeded with a handoff summary via `claude --bg`. User-invoked.
+- **[setup-ts-deep-modules](./setup-ts-deep-modules/SKILL.md)** — Wire dependency-cruiser into a TypeScript repo so each package is a deep module — implementation hidden in subfolders, reachable only through its entry-point files, tests exercising it through those. User-invoked.

skills/in-progress/setup-ts-deep-modules/SKILL.md

@@ -0,0 +1,102 @@
+---
+name: setup-ts-deep-modules
+description: Wire dependency-cruiser into a TypeScript repo so each package is a deep module — implementation hidden in subfolders, reachable only through its entry-point files. User-invoked.
+disable-model-invocation: true
+---
+
+# Setup TS Deep Modules
+
+Make every package in this repo a **deep module**: a lot of behaviour behind a small interface. A package's public surface is its **entry points** — the files at the package root — and everything in its subfolders is hidden. This skill installs [dependency-cruiser](https://github.com/sverweij/dependency-cruiser) and the rules that make the entry points the only way in, then proves the rules bite.
+
+For the vocabulary (deep module, interface, seam, depth), run the `/codebase-design` skill — use its language throughout.
+
+## The shape this enforces
+
+```
+src/packages/
+  <name>/
+    index.ts        ← an entry point (public). Import this from outside.
+    client.ts       ← another entry point. Packages may expose SEVERAL.
+    lib/            ← implementation: hidden from outside, free to import each other.
+    tests/          ← co-located tests + fixtures (a subfolder, so private).
+```
+
+The public surface is the package's **root files** — not one designated `index.ts`. By convention implementation lives in `lib/` and tests in `tests/`, giving every package the same two-folder shape. The rule itself is general, though: *anything* in *any* subfolder is private, so you never extend the config to add a folder.
+
+Four rules, all `error`:
+
+1. **Entry-point boundary** — code outside a package (app code or another package) may import only that package's entry points (its root files), never anything in its subfolders.
+2. **Intra-package freedom** — a package's own files import each other freely.
+3. **Tests through the entry points** — files under `<pkg>/tests/` may import any package's entry points and their own `tests/` fixtures, but never any package's subfolder internals (not even their own). Integration tests across packages are fine; deep imports are not.
+4. **No cycles** — no dependency cycles.
+
+**Entry points, not a barrel.** Because the public surface is *every* root file, a package can expose several small entry points (`index.ts`, `client.ts`, `server.ts`) instead of funnelling everything through one giant `index.ts`. Barrel files that re-export a whole subtree are discouraged — keep entry points small and hide implementation in subfolders.
+
+Layering (which packages may depend on which) is a *different* concern and is left as a commented stub in the config for this repo to fill in.
+
+## Steps
+
+### 1. Detect the environment
+
+- **Package manager** — `pnpm-lock.yaml` → pnpm, `yarn.lock` → yarn, `bun.lockb` → bun, else npm. Use it for every command below (`pnpm`/`yarn`/`npm run`/`bunx`).
+- **Packages root** — if `src/` exists use `src/packages`, else `packages`. Confirm the choice with the user if the repo already has a different obvious convention.
+- **Existing config** — check for a `.dependency-cruiser.*` file. If one exists, do **not** overwrite it: merge the four rules and the options in, and tell the user what you added.
+
+**Done when:** package manager, packages root, and existing-config status are all known.
+
+### 2. Install dependency-cruiser
+
+Install `dependency-cruiser` as a devDependency with the detected package manager.
+
+**Done when:** `dependency-cruiser` is in `devDependencies`.
+
+### 3. Write the config
+
+Copy [`dependency-cruiser.config.cjs`](./dependency-cruiser.config.cjs) to the repo root as `.dependency-cruiser.cjs`. Set `PACKAGES_ROOT` to the root detected in step 1. The rules are path-depth based and extension-agnostic, so nothing else needs adapting.
+
+**Done when:** `.dependency-cruiser.cjs` exists with the correct `PACKAGES_ROOT`, and the four forbidden rules are present.
+
+### 4. Wire it into the checks
+
+- Add a `lint:boundaries` script: `depcruise <packages-root>` (or `depcruise src`).
+- Fold it into the repo's umbrella check command — the one that already runs typecheck (e.g. a `check` / `ci` / `validate` script). Do **not** touch `tsconfig` or add path aliases.
+- If there is no umbrella script, add `lint:boundaries` and tell the user to include it in CI.
+
+**Done when:** `lint:boundaries` exists and runs as part of the same command as typecheck.
+
+### 5. Scaffold the example package
+
+Create a committed `<packages-root>/example/` as a copy-me template:
+
+- `index.ts` — an entry point. Export one function that delegates to an internal file (so the package is visibly *deep*, not a pass-through).
+- `lib/impl.ts` — an internal file in a **subfolder**, imported by `index.ts`, not reachable from outside.
+- `tests/example.test.ts` — imports **only** `../index` (an entry point), and asserts against the public function.
+
+Tell the user this is a starter template to copy or delete.
+
+**Done when:** the example package exists, exposes its behaviour through a root entry point, and hides `impl` in a subfolder.
+
+### 6. Prove the rules bite
+
+This is the completion criterion for the whole skill — a config that doesn't fail on a violation is worthless.
+
+1. Run `lint:boundaries`. It must **pass** on the clean example.
+2. Temporarily add a deep import to `tests/example.test.ts` (e.g. `import { thing } from "../lib/impl"`). Run `lint:boundaries` again — it must **fail** with `tests-through-entrypoints`.
+3. Revert the deep import. Run once more — it must **pass**.
+
+**Done when:** you have observed a pass, then a fail on the deep import, then a pass again. If step 2 does not fail, the rules are not wired correctly — fix before finishing.
+
+### 7. Document the convention
+
+Write a `README.md` **in the packages folder** (`<packages-root>/README.md`) — next to the packages it governs — covering: the `src/packages/<name>/` layout (entry points at the root, `lib/` for implementation, `tests/` for tests), "import only through a package's entry points (its root files)", and how to run `lint:boundaries`. **Discourage barrel files** explicitly — expose several small entry points instead of re-exporting a whole subtree through one index. Keep it to the copy-me snippet plus the four rules in one paragraph each.
+
+Then add a **context pointer** to it from the repo's agent-instructions file — `CLAUDE.md` if present, else `AGENTS.md` (create `AGENTS.md` if neither exists). One line is enough, e.g. `Packages are deep modules — see [src/packages/README.md](./src/packages/README.md) before adding or importing one.` This is what makes an agent discover the boundary rule instead of tripping over it.
+
+**Done when:** `<packages-root>/README.md` exists and discourages barrels, and the repo's `CLAUDE.md`/`AGENTS.md` links to it.
+
+## Notes
+
+- The config's `$1` back-references (dependency-cruiser's group matching) are what let a package reach its own internals while outsiders can't — don't flatten them into separate per-package rules.
+- Public vs private is decided by **depth**: a package's root files are entry points; anything in a subfolder is private. The conventional subfolders are `lib/` (implementation) and `tests/`, but the rule doesn't hardcode them — any subfolder is private, so a new folder never needs a config change. Adding an entry point is just adding a root file — no barrel.
+- Packages are **flat**: one tier of immediate children under the root. A package's internals may nest as deep as you like; a package may not contain another package.
+- Use `.cjs` (not `.js`) so the config's `module.exports` works even in `"type": "module"` repos.

skills/in-progress/setup-ts-deep-modules/dependency-cruiser.config.cjs

@@ -0,0 +1,95 @@
+// @ts-check
+// Deep-module enforcement for dependency-cruiser.
+//
+// Each package under the packages root is a DEEP MODULE: a lot of behaviour
+// behind a small interface. A package's PUBLIC SURFACE is its ENTRY POINTS —
+// the files at the package root. Implementation lives in SUBFOLDERS and is
+// private — by convention `lib/` for implementation and `tests/` for tests,
+// though any subfolder is private. A package may expose several small entry
+// points (index.ts, client.ts, server.ts, …) — prefer that over one giant
+// barrel index.
+//
+// The only thing you should ever need to edit here is PACKAGES_ROOT.
+
+/** Where packages live. One immediate child dir per package (flat, no nesting). */
+const PACKAGES_ROOT = "src/packages";
+
+// --- derived patterns (no need to edit) -------------------------------------
+const R = PACKAGES_ROOT;
+/**
+ * A package's private internals: anything nested inside a package subfolder.
+ * The package's root files are its entry points and are NOT matched here —
+ * they stay importable from outside.
+ */
+const PACKAGE_INTERNALS = `^${R}/[^/]+/[^/]+/`;
+
+/** @type {import('dependency-cruiser').IConfiguration} */
+module.exports = {
+  forbidden: [
+    {
+      name: "entrypoint-boundary-from-app",
+      comment:
+        "App/root code may import a package's entry points (its root files), but nothing inside its subfolders.",
+      severity: "error",
+      from: { pathNot: `^${R}/` }, // importer is NOT inside any package
+      to: { path: PACKAGE_INTERNALS },
+    },
+    {
+      name: "entrypoint-boundary-across-packages",
+      comment:
+        "A package's own files import each other freely, but may reach OTHER packages only through their entry points — never their internals.",
+      severity: "error",
+      // importer is inside a package ($1), but is not a test file
+      from: { path: `^${R}/([^/]+)/`, pathNot: `^${R}/[^/]+/tests/` },
+      to: {
+        path: PACKAGE_INTERNALS,
+        pathNot: `^${R}/$1/`, // same package → intra-package freedom
+      },
+    },
+    {
+      name: "tests-through-entrypoints",
+      comment:
+        "A package's tests exercise it through its entry points like everyone else: they may import any package's entry points and their own tests/ fixtures, but never any package's internals — not even their own.",
+      severity: "error",
+      from: { path: `^${R}/([^/]+)/tests/` }, // a test file, in package $1
+      to: {
+        path: PACKAGE_INTERNALS,
+        pathNot: `^${R}/$1/tests/`, // own tests/ fixtures → allowed
+      },
+    },
+    {
+      name: "tests-folder-is-private",
+      comment:
+        "A package's tests/ folder is reachable only from tests — nothing else may import fixtures.",
+      severity: "error",
+      from: { pathNot: `^${R}/[^/]+/tests/` }, // importer is not itself a test
+      to: { path: `^${R}/[^/]+/tests/` },
+    },
+    {
+      name: "no-circular",
+      comment: "No dependency cycles. Scope to `^${R}/` if you want to allow cycles outside packages.",
+      severity: "error",
+      from: {},
+      to: { circular: true },
+    },
+
+    // --- Layering (optional, off by default) ----------------------------------
+    // Interface-hiding controls HOW you import (through the entry points).
+    // Layering controls WHICH packages may depend on which. Add your own rules
+    // here, e.g.:
+    //
+    // {
+    //   name: "ui-may-not-depend-on-billing",
+    //   severity: "error",
+    //   from: { path: `^${R}/ui/` },
+    //   to:   { path: `^${R}/billing/` },
+    // },
+  ],
+  options: {
+    doNotFollow: { path: "node_modules" },
+    tsConfig: { fileName: "tsconfig.json" },
+    enhancedResolveOptions: {
+      extensions: [".ts", ".tsx", ".js", ".jsx", ".json"],
+    },
+  },
+};