fix: correct stale claims in the router and the READMEs

Coherence pass over the rewritten docs set. The recurring defect was a
refactor whose removals landed and whose replacements did not.

- tdd: restore the pointer to /codebase-design that the v1.0 changelog
  and ask-matt both claim exists. The inline deep-module notes were
  deleted then; nothing replaced them.
- ask-matt: /grilling and /resolving-merge-conflicts were missing from
  the router entirely. Split grill-me from grill-with-docs on the
  working directory rather than on whether the subject is code.
- READMEs: wayfinder maps decision tickets, not investigation tickets;
  the diagnosing-bugs loop starts by building a loop that goes red;
  improve-codebase-architecture is a survey, not a rescue; grilling
  resolves a design tree and is the primitive behind five skills.
- Drop the /implement reliability claim from the implement and tdd
  pages.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/engineering/grill-with-docs.md

@@ -12,7 +12,7 @@ Reach for it at the start of a change, in a repo, when the plan is still fuzzy a
 
 | What you have | Reach for |
 | --- | --- |
-| An idea, no repo — or a subject that isn't code | [grill-me](https://aihero.dev/skills-grill-me) |
+| You aren't working in a working directory at all | [grill-me](https://aihero.dev/skills-grill-me) |
 | A repo, and a change you can settle in one session | `grill-with-docs` |
 | An effort too big to hold in one session — a greenfield build, a large feature | [wayfinder](https://aihero.dev/skills-wayfinder) |
 | A repo with no domain docs at all, and no particular feature in mind | `grill-with-docs`, aimed at the repo rather than a change |