chore(skill): Own tttk-authoring in the repo

Move the skill to .agents/skills/tttk-authoring and point gen:skill-reference at it. Activate it by symlinking ~/.agents/skills/tttk-authoring to the in-repo copy.

Extend the reference drift guard to cover the skill copy now that it is versioned, and document activation in docs/development.md.

docs/check-plan.md

@@ -337,7 +337,7 @@ Exit code is the contract that makes `check` usable in an agent loop.
 11. **Reference generator** — `scripts/generate-reference.ts` renders the
     `known.ts` catalog to markdown. Canonical output `docs/reference.md`
     (versioned, testable); `--out` can target the skill copy at
-    `~/.agents/skills/tttk-authoring/reference.md`. Add `npm run
+    `.agents/skills/tttk-authoring/reference.md`. Add `npm run
     gen:reference`.
 12. **Drift guard** — a test that greps `customElement("md-*")` call sites
     under `src/components` and asserts each appears in `known.ts` (and vice
@@ -363,7 +363,7 @@ Exit code is the contract that makes `check` usable in an agent loop.
 - Update `docs/cli.md`: replace the `compile` section with `check`; document
   codes, `--json`, exit codes.
 - Update `README.md`.
-- Update `~/.agents/skills/tttk-authoring/SKILL.md`: the verification step
+- Update `.agents/skills/tttk-authoring/SKILL.md`: the verification step
   becomes **`npx tttk check ./content`**, and the workflow becomes
   write → check → fix. Remove the "compile is a no-op stub" note. Cross-link
   the code catalog (`reference.md`) with the diagnostic codes.