feat: add the research skill

A small, model-invoked skill that spins up a background agent to
investigate a question against high-trust primary sources, then leaves a
single cited Markdown file wherever the repo keeps such notes.

Wires it into the plugin set, both READMEs (Model-invoked), a docs page,
and the ask-matt router, per the repo's promoted-skill conventions.

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

.changeset/research-skill.md

@@ -0,0 +1,5 @@
+---
+"mattpocock-skills": minor
+---
+
+Add the **`research`** skill — a small, model-invoked skill that spins up a **background agent** to investigate a question against **primary sources** (official docs, source code, specs, first-party APIs), then leaves a single cited Markdown file wherever the repo keeps such notes. It's delegable reading legwork: you keep working while it reads, and get back a document to grill, plan, or design against. Listed in the top-level and Engineering READMEs (Model-invoked), added to `.claude-plugin/plugin.json`, given a docs page at `docs/engineering/research.md`, and routed as a Standalone in `ask-matt`.

.changeset/tdd-reference-only-seams.md

@@ -0,0 +1,7 @@
+---
+"mattpocock-skills": patch
+---
+
+Reshape the `tdd` skill into reference-only. The red → green → refactor loop is anchored by leading words the model already holds, so the step-by-step Workflow was largely restating the loop and duplicating the horizontal-slicing anti-pattern. Dropped the Workflow and per-cycle checklist; folded their one durable idea — vertical slices / tracer bullets — into the Anti-patterns section and a short Rules-of-the-loop list. Introduced **seam** as the leading word for where tests go, collapsing the old Philosophy "public interfaces" prose and the Planning "confirm interface / behaviors" handshake into one rule: test only at pre-agreed seams, confirmed with the user before any test is written.
+
+Also dropped the refactor stage — TDD is now red → green, not red → green → refactor. Refactoring belongs to the review stage, not the implementation loop, so the refactor rule and `refactoring.md` were removed (its home is the `review` skill).

.claude-plugin/plugin.json

@@ -11,6 +11,7 @@
     "./skills/engineering/to-issues",
     "./skills/engineering/to-prd",
     "./skills/engineering/prototype",
+    "./skills/engineering/research",
     "./skills/engineering/domain-modeling",
     "./skills/engineering/codebase-design",
     "./skills/engineering/code-review",

README.md

@@ -161,6 +161,7 @@ Skills I use daily for code work.
 
 - **[prototype](./skills/engineering/prototype/SKILL.md)** — Build a throwaway prototype to answer a design question — a runnable terminal app for state/logic questions, or several radically different UI variations toggleable from one route.
 - **[diagnosing-bugs](./skills/engineering/diagnosing-bugs/SKILL.md)** — Disciplined diagnosis loop for hard bugs and performance regressions: reproduce → minimise → hypothesise → instrument → fix → regression-test.
+- **[research](./skills/engineering/research/SKILL.md)** — Investigate a question against high-trust primary sources and capture the findings as a cited Markdown file in the repo, run as a background agent.
 - **[tdd](./skills/engineering/tdd/SKILL.md)** — Test-driven development with a red-green-refactor loop. Builds features or fixes bugs one vertical slice at a time.
 - **[domain-modeling](./skills/engineering/domain-modeling/SKILL.md)** — Actively build and sharpen a project's domain model — challenge terms against the glossary, stress-test with edge-case scenarios, and update `CONTEXT.md` and ADRs inline.
 - **[codebase-design](./skills/engineering/codebase-design/SKILL.md)** — Shared discipline and vocabulary for designing deep modules: a lot of behaviour behind a small interface, placed at a clean seam, testable through that interface.

docs/engineering/research.md

@@ -0,0 +1,29 @@
+Quickstart:
+
+```bash
+npx skills add mattpocock/skills --skill=research
+```
+
+```bash
+npx skills update research
+```
+
+[Source](https://github.com/mattpocock/skills/tree/main/skills/engineering/research)
+
+## What it does
+
+`research` answers a question by reading the sources that own the answer and leaving a cited Markdown file behind. It works only from **primary sources** — official docs, source code, specs, first-party APIs — never a secondary write-up of them, so what it saves is traceable back to something authoritative rather than a summary of a summary.
+
+## When to reach for it
+
+Type `/research`, or the agent reaches for it automatically when a task turns into reading legwork.
+
+Reach for it when the next step is *finding something out* — how an API behaves, what a spec actually says, whether a claim holds — and you'd rather not stall your own thread doing the reading. For sharpening a plan by interview instead of by reading, use [grilling](https://aihero.dev/skills-grilling); for exploring what to build with throwaway code, use [prototype](https://aihero.dev/skills-prototype).
+
+## Delegated legwork
+
+The defining move is that the reading runs as a **background agent**. You keep working; it goes off, follows each claim back to its primary source, and drops a single cited Markdown file into wherever the repo keeps such notes. Research is legwork you delegate, not thinking you outsource — you get back a document to react to, with its sources attached.
+
+## Where it fits
+
+A reach-for-it-anytime standalone that feeds the thinking skills: the file it produces is something to grill, plan, or design against, so it sits upstream of work like [grilling](https://aihero.dev/skills-grilling) and [to-prd](https://aihero.dev/skills-to-prd) rather than in the build chain. For the whole map, see [ask-matt](https://aihero.dev/skills-ask-matt).

skills/engineering/README.md

@@ -21,6 +21,7 @@ Model- or user-reachable (rich trigger phrasing so the model can reach for them)
 - **[prototype](./prototype/SKILL.md)** — Build a throwaway prototype to answer a design question: a runnable terminal app for state/logic, or several toggleable UI variations.
 
 - **[diagnosing-bugs](./diagnosing-bugs/SKILL.md)** — Disciplined diagnosis loop for hard bugs and performance regressions: reproduce → minimise → hypothesise → instrument → fix → regression-test.
+- **[research](./research/SKILL.md)** — Investigate a question against high-trust primary sources and capture the findings as a cited Markdown file in the repo, run as a background agent.
 - **[tdd](./tdd/SKILL.md)** — Test-driven development with a red-green-refactor loop. Builds features or fixes bugs one vertical slice at a time.
 - **[domain-modeling](./domain-modeling/SKILL.md)** — Actively build and sharpen a project's domain model — challenge terms, stress-test with scenarios, update `CONTEXT.md` and ADRs inline.
 - **[codebase-design](./codebase-design/SKILL.md)** — Shared discipline and vocabulary for designing deep modules: small interfaces, clean seams, testable through the interface.

skills/engineering/ask-matt/SKILL.md

@@ -65,6 +65,7 @@ Off the main flow entirely.
 
 - **`/grill-me`** — the same relentless interview as `/grill-with-docs`, but for when you have **no codebase**. Stateless: it saves nothing locally, builds no `CONTEXT.md`. Reach for it to sharpen any plan or design that doesn't live in a repo.
 - **`/prototype`** — a small, throwaway program that answers one design question: does this state model feel right, or what should this UI look like. Throwaway from day one — keep the answer, delete the code. It's the detour in step 2 of the main flow, but reach for it any time a design question is hard to settle on paper.
+- **`/research`** — delegate reading legwork to a **background agent**: it investigates a question against **primary sources**, then leaves a cited Markdown file in the repo. Keep working while it reads. The file it produces is something to take *into* the main flow at `/grill-with-docs` — research feeds the thinking, it doesn't replace it.
 - **`/teach`** — learn a concept over multiple sessions, using the current directory as a stateful workspace.
 - **`/writing-great-skills`** — reference for writing and editing skills well.
 

skills/engineering/research/SKILL.md

@@ -0,0 +1,12 @@
+---
+name: research
+description: Investigate a question against high-trust primary sources and capture the findings as a Markdown file in the repo. Use when the user wants a topic researched, docs or API facts gathered, or reading legwork delegated to a background agent.
+---
+
+Spin up a **background agent** to do the research, so you keep working while it reads.
+
+Its job:
+
+1. Investigate the question against **primary sources** — official docs, source code, specs, first-party APIs — not a secondary write-up of them. Follow every claim back to the source that owns it.
+2. Write the findings to a single Markdown file, citing each claim's source.
+3. Save it where the repo already keeps such notes; match the existing convention, and if there is none, put it somewhere sensible and say where.

skills/engineering/tdd/SKILL.md

@@ -5,107 +5,32 @@ description: Test-driven development. Use when the user wants to build features
 
 # Test-Driven Development
 
-## Philosophy
+TDD is the red → green loop. This skill is the reference that makes that loop produce tests worth keeping: what a good test is, where tests go, the anti-patterns, and the rules of the loop. Every section applies on every cycle — consult them before and during the loop, not after.
 
-**Core principle**: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.
+When exploring the codebase, read `CONTEXT.md` (if it exists) so test names and interface vocabulary match the project's domain language, and respect ADRs in the area you're touching.
 
-**Good tests** are integration-style: they exercise real code paths through public APIs. They describe _what_ the system does, not _how_ it does it. A good test reads like a specification - "user can checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure.
+## What a good test is
 
-**Bad tests** are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means (like querying a database directly instead of using the interface). The warning sign: your test breaks when you refactor, but behavior hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behavior.
-
-**Tautological tests** restate the implementation inside the assertion, so they pass by construction and give zero confidence. When the expected value is computed the way the code computes it — `expect(add(a, b)).toBe(a + b)`, snapshotting a figure you derived by hand the same way the code does, asserting a constant equals itself — the test can never disagree with the code: break the code wrong and the assertion breaks wrong with it. The expected value must come from an independent source of truth — a known-good literal, a worked example, the spec.
+Tests verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't. A good test reads like a specification — "user can checkout with valid cart" tells you exactly what capability exists — and survives refactors because it doesn't care about internal structure.
 
 See [tests.md](tests.md) for examples and [mocking.md](mocking.md) for mocking guidelines.
 
-## Anti-Pattern: Horizontal Slices
+## Seams — where tests go
 
-**DO NOT write all tests first, then all implementation.** This is "horizontal slicing" - treating RED as "write all tests" and GREEN as "write all code."
+A **seam** is the public boundary you test at: the interface where you observe behavior without reaching inside. Tests live at seams, never against internals.
 
-This produces **crap tests**:
+**Test only at pre-agreed seams.** Before writing any test, write down the seams under test and confirm them with the user. No test is written at an unconfirmed seam. You can't test everything — agreeing the seams up front is how testing effort lands on the critical paths and complex logic instead of every edge case.
 
-- Tests written in bulk test _imagined_ behavior, not _actual_ behavior
-- You end up testing the _shape_ of things (data structures, function signatures) rather than user-facing behavior
-- Tests become insensitive to real changes - they pass when behavior breaks, fail when behavior is fine
-- You outrun your headlights, committing to test structure before understanding the implementation
+Ask: "What's the public interface, and which seams should we test?"
 
-**Correct approach**: Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle. Because you just wrote the code, you know exactly what behavior matters and how to verify it.
+## Anti-patterns
 
-```
-WRONG (horizontal):
-  RED:   test1, test2, test3, test4, test5
-  GREEN: impl1, impl2, impl3, impl4, impl5
+- **Implementation-coupled** — mocks internal collaborators, tests private methods, or verifies through a side channel (querying the database instead of using the interface). The tell: the test breaks when you refactor but behavior hasn't changed.
+- **Tautological** — the assertion recomputes the expected value the way the code does (`expect(add(a, b)).toBe(a + b)`, a snapshot derived by hand the same way, a constant asserted equal to itself), so it passes by construction and can never disagree with the code. Expected values must come from an independent source of truth — a known-good literal, a worked example, the spec.
+- **Horizontal slicing** — writing all tests first, then all implementation. Bulk tests verify _imagined_ behavior: you test the _shape_ of things rather than user-facing behavior, the tests go insensitive to real changes, and you commit to test structure before understanding the implementation. Work in **vertical slices** instead — one test → one implementation → repeat, each test a **tracer bullet** that responds to what the last cycle taught you.
 
-RIGHT (vertical):
-  RED→GREEN: test1→impl1
-  RED→GREEN: test2→impl2
-  RED→GREEN: test3→impl3
-  ...
-```
+## Rules of the loop
 
-## Workflow
-
-### 1. Planning
-
-When exploring the codebase, read `CONTEXT.md` (if it exists) so that test names and interface vocabulary match the project's domain language, and respect ADRs in the area you're touching.
-
-Before writing any code:
-
-- [ ] Confirm with user what interface changes are needed
-- [ ] Confirm with user which behaviors to test (prioritize)
-- [ ] Identify opportunities for deep modules (small interface, deep implementation) — run the `/codebase-design` skill for the vocabulary and the testability checks
-- [ ] List the behaviors to test (not implementation steps)
-- [ ] Get user approval on the plan
-
-Ask: "What should the public interface look like? Which behaviors are most important to test?"
-
-**You can't test everything.** Confirm with the user exactly which behaviors matter most. Focus testing effort on critical paths and complex logic, not every possible edge case.
-
-### 2. Tracer Bullet
-
-Write ONE test that confirms ONE thing about the system:
-
-```
-RED:   Write test for first behavior → test fails
-GREEN: Write minimal code to pass → test passes
-```
-
-This is your tracer bullet - proves the path works end-to-end.
-
-### 3. Incremental Loop
-
-For each remaining behavior:
-
-```
-RED:   Write next test → fails
-GREEN: Minimal code to pass → passes
-```
-
-Rules:
-
-- One test at a time
-- Only enough code to pass current test
-- Don't anticipate future tests
-- Keep tests focused on observable behavior
-
-### 4. Refactor
-
-After all tests pass, look for [refactor candidates](refactoring.md):
-
-- [ ] Extract duplication
-- [ ] Deepen modules (move complexity behind simple interfaces)
-- [ ] Apply SOLID principles where natural
-- [ ] Consider what new code reveals about existing code
-- [ ] Run tests after each refactor step
-
-**Never refactor while RED.** Get to GREEN first.
-
-## Checklist Per Cycle
-
-```
-[ ] Test describes behavior, not implementation
-[ ] Test uses public interface only
-[ ] Test would survive internal refactor
-[ ] Expected values are independent literals, not recomputed from the code
-[ ] Code is minimal for this test
-[ ] No speculative features added
-```
+- **Red before green.** Write the failing test first, then only enough code to pass it. Don't anticipate future tests or add speculative features.
+- **One slice at a time.** One seam, one test, one minimal implementation per cycle.
+- **Refactoring is not part of the loop.** It belongs to the review stage (see the `review` skill), not the red → green implementation cycle.

skills/engineering/tdd/refactoring.md

@@ -1,10 +0,0 @@
-# Refactor Candidates
-
-After TDD cycle, look for:
-
-- **Duplication** → Extract function/class
-- **Long methods** → Break into private helpers (keep tests on public interface)
-- **Shallow modules** → Combine or deepen
-- **Feature envy** → Move logic to where data lives
-- **Primitive obsession** → Introduce value objects
-- **Existing code** the new code reveals as problematic