ask-matt: map the full skill set, add router maintenance rule

Ask Matt was missing five skills that now exist and are user-reachable:
tdd (the red-green engine implement drives), diagnosing-bugs (there was
no route for "something's broken"), domain-modeling and codebase-design
(the two vocabulary references), and grilling (the shared primitive).

- SKILL.md: add a "Something's broken" on-ramp for diagnosing-bugs, a
  "Vocabulary underneath" section for domain-modeling/codebase-design,
  weave tdd into the main flow, flesh out prototype, broaden the
  description from "user-invoked skills" to "the skills".
- docs/engineering/ask-matt.md: re-sync the framing to match.
- CLAUDE.md: add a maintenance rule so any future skill change triggers
  an Ask Matt re-check, beside the existing docs-page re-sync rule.

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

CLAUDE.md

@@ -17,4 +17,6 @@ Skills in `engineering/` and `productivity/` also have a human-facing docs page
 
 Every `SKILL.md` is either user-invoked (`disable-model-invocation: true`, reachable only by the human) or model-invoked (model- or user-reachable). See [.agents/invocation.md](./.agents/invocation.md).
 
+[`ask-matt`](./skills/engineering/ask-matt/SKILL.md) is the router that maps every user-reachable skill and how they relate. The same trigger that re-syncs a docs page applies to it: whenever you add, rename, remove, or change how a user-reachable skill fits the flows, re-read `ask-matt`'s `SKILL.md` and update it so the map stays accurate — a new skill it never mentions, or a stale one it still routes to, is a router that lies.
+
 To (re)link every skill into the local harness skill directories (`~/.claude/skills`, `~/.agents/skills`), run `scripts/link-skills.sh`. Each entry is a symlink into this repo, so a `git pull` keeps installed skills current; re-run the script after adding, removing, or renaming a skill.