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>

skills/engineering/implement/SKILL.md

@@ -1,10 +1,10 @@
 ---
 name: implement
-description: "Implement a piece of work based on a PRD or set of issues."
+description: "Implement a piece of work based on a spec or set of tickets."
 disable-model-invocation: true
 ---
 
-Implement the work described by the user in the PRD or issues.
+Implement the work described by the user in the spec or tickets.
 
 Use /tdd where possible, at pre-agreed seams.