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>

docs/engineering/ask-matt.md

@@ -14,7 +14,7 @@ npx skills update ask-matt
 
 `ask-matt` is the router over the skills in this repo. You describe the situation you're in; it tells you which skill or flow fits and in what order to run them.
 
-It **does no work itself**. It doesn't grill, write a PRD, or fix anything — it only orients. It exists for the **user-invoked** skills above all: nothing fires those for you, so *you* have to remember they exist, and `ask-matt` is the memory you offload that to. It also points at the model-invoked skills you'd reach for by name — `/tdd`, `/diagnosing-bugs`, `/prototype`, `/code-review`, and the two vocabulary references, `/domain-modeling` and `/codebase-design`. It answers "which one, and when", then hands you off to the skill that actually does the job.
+It **does no work itself**. It doesn't grill, write a spec, or fix anything — it only orients. It exists for the **user-invoked** skills above all: nothing fires those for you, so *you* have to remember they exist, and `ask-matt` is the memory you offload that to. It also points at the model-invoked skills you'd reach for by name — `/tdd`, `/diagnosing-bugs`, `/prototype`, `/code-review`, and the two vocabulary references, `/domain-modeling` and `/codebase-design`. It answers "which one, and when", then hands you off to the skill that actually does the job.
 
 ## When to reach for it
 
@@ -24,7 +24,7 @@ Reach for it whenever you're unsure which skill or flow a situation calls for: y
 
 ## Flows, not just skills
 
-The idea `ask-matt` gives you to think with is the **flow** — a path *through* the skills rather than a single one. Most work runs along one **main flow** (idea → ship: grill → PRD → issues → implement → review), two **on-ramps** merge onto it (a triage lane for incoming bugs and requests; a codebase-health lane that generates ideas), and everything else is a **standalone** you reach for on its own. Ask a question and you get placed on the right flow, at the right step — not just handed a tool.
+The idea `ask-matt` gives you to think with is the **flow** — a path *through* the skills rather than a single one. Most work runs along one **main flow** (idea → ship: grill → spec → tickets → implement → review), two **on-ramps** merge onto it (a triage lane for incoming bugs and requests; a codebase-health lane that generates ideas), and everything else is a **standalone** you reach for on its own. Ask a question and you get placed on the right flow, at the right step — not just handed a tool.
 
 ## Where it fits