feat(to-plan): add sequential plan skill

Add a user-invoked `to-plan` skill — the sequential, HITL sibling of
`to-issues`. Both slice work into tracer-bullet vertical slices, but
`to-plan` produces one ordered sequence you drive by hand, one phase per
fresh context, rather than parallel independently-grabbable issues.

It publishes to the tracker `/setup-matt-pocock-skills` configured, shaped
to it: a single sequential `plan.md` for a local tracker, or a parent
issue with ordered sub-issues (native sub-issues where supported) for a
real one.

Registers the skill everywhere CLAUDE.md requires (plugin.json, top-level
and bucket READMEs, docs page), updates the `ask-matt` router's main-flow
fork to offer both to-issues and to-plan, and adds a changeset.

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

docs/engineering/to-plan.md

@@ -0,0 +1,53 @@
+Quickstart:
+
+```bash
+npx skills add mattpocock/skills --skill=to-plan
+```
+
+```bash
+npx skills update to-plan
+```
+
+[Source](https://github.com/mattpocock/skills/tree/main/skills/engineering/to-plan)
+
+## What it does
+
+`to-plan` turns a PRD, spec, or the current conversation into a **sequential** plan — an ordered list of phases you work through one at a time, in a fresh context per phase.
+
+Every phase is a **tracer bullet** — a thin *vertical* slice that cuts through all integration layers end-to-end (schema, API, UI, tests), never a horizontal slice of one layer. A completed slice is demoable or verifiable on its own, and is sized to fit a single fresh context window.
+
+## When to reach for it
+
+You invoke this by typing `/to-plan` — the agent won't reach for it on its own.
+
+Reach for it once you have an agreed plan or a written spec and you want it turned into a sequence of phases you'll build **by hand, one at a time**. Point it at the conversation, or pass a PRD or issue reference and it reads that first. If the change hasn't been written up as a spec yet, produce one first — for that, use [to-prd](https://aihero.dev/skills-to-prd).
+
+## Sequential, not parallel
+
+`to-plan` is the sequential, HITL sibling of [to-issues](https://aihero.dev/skills-to-issues). Both slice work into tracer bullets, but they're for different workflows:
+
+- **`to-issues`** produces *independently-grabbable* issues, so parallel agents can pick them up at once (it applies the ready-for-agent label).
+- **`to-plan`** produces one *ordered sequence* you drive yourself, phase by phase, staying in the loop — no ready-for-agent label.
+
+## Publishes to your configured tracker
+
+Like the other skills, `to-plan` publishes through the tracker [setup-matt-pocock-skills](https://aihero.dev/skills-setup-matt-pocock-skills) configured, and it adapts the shape to that tracker:
+
+- **Local files** → a single sequential `plan.md` in the repo root, all phases in order.
+- **A real issue tracker (GitHub, Linear)** → a parent issue for the whole plan, with one sub-issue per phase, in order — using the platform's native sub-issue relationship where it has one, otherwise sequential issues each blocked by the previous.
+
+## Vertical slices, not horizontal ones
+
+The whole skill turns on one distinction. A **horizontal** slice ships one layer of the change — all the schema, or all the API — and nothing works until every layer lands. A **vertical** slice, the tracer bullet, ships one narrow path through *every* layer at once, so it can be demoed the moment it's done.
+
+Before slicing, `to-plan` looks for prefactoring — "make the change easy, then make the easy change" — and orders that work first. It then quizzes you on the breakdown (granularity, ordering, what to merge or split) before publishing anything.
+
+## Where it fits
+
+`to-plan` is the sequential branch of the main build chain:
+
+```txt
+grill-with-docs → to-prd → to-plan → implement → code-review
+```
+
+It sits between [to-prd](https://aihero.dev/skills-to-prd), which hands it a settled spec to slice against, and [implement](https://aihero.dev/skills-implement), which builds each phase, 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 one phase 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.