refactor: unify planning skills into /to-spec + /to-tickets

Executes the `unify-to-tickets` decision (personal-wiki #23), reworking
this PR from "add /to-plan" into the full planning-skills unification.

- Rename /to-prd → /to-spec. "spec" is now the single through-line term
  (the skill still opens with "you may know this document as a PRD" for
  discoverability).
- Merge /to-plan + /to-issues into one /to-tickets skill; delete
  /to-issues. /to-tickets emits a set of tickets, each declaring its
  blocking edges — one medium-agnostic artifact that reads as a
  sequential plan (local `tickets.md`, edges as text) or a parallelizable
  DAG (real tracker, native blocking links). The edges live in the ticket
  either way; the medium only decides whether anything acts on them in
  parallel.
- ask-matt now routes idea → /to-spec → /to-tickets → /implement.
- Sweep every cross-reference (docs pages, chain diagrams, CONTEXT.md,
  ADRs, .agents, setup templates) to the new names and vocabulary.
- Rewrite the changeset to describe the unification.

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

.agents/writing-docs.md

@@ -30,7 +30,7 @@ npx skills update <name>
 
 ## What it does
 
-One or two plain-language paragraphs. Lead with the skill's one-sentence job, then state the **defining constraint** — the single fact that makes this skill behave differently from the obvious default (for `to-prd`: it does not interview the user again, it synthesises what is already known). Write it as a plain declarative sentence — never a labelled aside like "The defining constraint:" or "The key thing:"; the formula reads as filler. This line is the most valuable on the page; never omit it.
+One or two plain-language paragraphs. Lead with the skill's one-sentence job, then state the **defining constraint** — the single fact that makes this skill behave differently from the obvious default (for `to-spec`: it does not interview the user again, it synthesises what is already known). Write it as a plain declarative sentence — never a labelled aside like "The defining constraint:" or "The key thing:"; the formula reads as filler. This line is the most valuable on the page; never omit it.
 
 ## When to reach for it
 
@@ -41,7 +41,7 @@ How and when you reach for the skill — two beats, both effectively always pres
 
 ## Prerequisites
 
-Optional — include only when the skill needs something in place to be functional; omit the heading entirely otherwise. Covers: a **workspace it writes into** (a stateful skill like `grill-with-docs` writes `CONTEXT.md` and ADRs; `teach` builds a whole directory — say what it writes and where), **prior setup** (`triage`/`to-prd`/`to-issues` need `setup-matt-pocock-skills` to have configured an issue tracker), or **repo-specific tooling**. A stateless skill that runs anywhere has no prerequisites — drop the section.
+Optional — include only when the skill needs something in place to be functional; omit the heading entirely otherwise. Covers: a **workspace it writes into** (a stateful skill like `grill-with-docs` writes `CONTEXT.md` and ADRs; `teach` builds a whole directory — say what it writes and where), **prior setup** (`triage`/`to-spec`/`to-tickets` need `setup-matt-pocock-skills` to have configured an issue tracker), or **repo-specific tooling**. A stateless skill that runs anywhere has no prerequisites — drop the section.
 
 ## <free-form middle>
 
@@ -51,13 +51,13 @@ The single non-negotiable: **surface the skill's leading word / defining idea**
 
 ## It's working if
 
-Optional. A short, checkable list of the observable signals that tell the reader the skill is actually doing its job — what they should see when it fires, and by absence when it hasn't. Include it when a skill has crisp tells (e.g. `to-prd` writes without re-interviewing you; a leading word reappearing in the trace); omit the heading when the signals are vague. A few bullets, no more.
+Optional. A short, checkable list of the observable signals that tell the reader the skill is actually doing its job — what they should see when it fires, and by absence when it hasn't. Include it when a skill has crisp tells (e.g. `to-spec` writes without re-interviewing you; a leading word reappearing in the trace); omit the heading when the signals are vague. A few bullets, no more.
 
 ## Where it fits
 
 Always present. Situate the skill in the system in a sentence or two:
 
-- **Role.** Name it: a **chain step** (`grill-with-docs → to-prd → to-issues → implement → code-review`), a **run-once setup** (`setup-matt-pocock-skills`), **periodic maintenance** (`improve-codebase-architecture`, "every few days"), or a **reach-for-it-anytime standalone** (`diagnosing-bugs`, `prototype`, `handoff`). A standalone's map is one honest sentence — far better than omitting the section.
+- **Role.** Name it: a **chain step** (`grill-with-docs → to-spec → to-tickets → implement → code-review`), a **run-once setup** (`setup-matt-pocock-skills`), **periodic maintenance** (`improve-codebase-architecture`, "every few days"), or a **reach-for-it-anytime standalone** (`diagnosing-bugs`, `prototype`, `handoff`). A standalone's map is one honest sentence — far better than omitting the section.
 - **Neighbours.** The one or two siblings that matter, each with a because-clause, linked absolutely.
 - **The map.** Point to [ask-matt](https://aihero.dev/skills-ask-matt), the router over the whole set, so this page stays a node and never has to redraw the graph.