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>

.claude-plugin/plugin.json

@@ -8,9 +8,8 @@
     "./skills/engineering/improve-codebase-architecture",
     "./skills/engineering/setup-matt-pocock-skills",
     "./skills/engineering/tdd",
-    "./skills/engineering/to-issues",
-    "./skills/engineering/to-plan",
-    "./skills/engineering/to-prd",
+    "./skills/engineering/to-spec",
+    "./skills/engineering/to-tickets",
     "./skills/engineering/implement",
     "./skills/engineering/prototype",
     "./skills/engineering/research",