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/skill-reference-plan.md

@@ -10,6 +10,10 @@ skill's `reference.md` is now generated and byte-identical to
 `docs/reference.md`, with the extra content moved to `layout.md`, `yaml-tag.md`,
 `csv.md`, and `markdown.md`.
 
+The skill is now **owned by the repo** at `.agents/skills/tttk-authoring/` and
+activated by symlinking `~/.agents/skills/tttk-authoring` to it (see
+`docs/development.md` § Agent skills).
+
 ## 1. Goal
 
 Make the `tttk-authoring` skill's `reference.md` a **generated** file — the same
@@ -19,10 +23,10 @@ hand-maintained element list, and no content lost in the swap.
 
 ## 2. Current state
 
-The skill (`~/.agents/skills/tttk-authoring/`) has a hand-written
-`reference.md` (~465 lines) and `SKILL.md`. The repo now generates
-`docs/reference.md` from `src/cli/validate/known.ts` via `npm run
-gen:reference`.
+The skill (originally `~/.agents/skills/tttk-authoring/`, now
+`.agents/skills/tttk-authoring/`) had a hand-written `reference.md` (~465 lines)
+and `SKILL.md`. The repo generates `docs/reference.md` from
+`src/cli/validate/known.ts` via `npm run gen:reference`.
 
 What the generated doc already covers:
 
@@ -109,7 +113,7 @@ generator exists to prevent.
 `package.json`:
 
 - `gen:reference` — unchanged (writes `docs/reference.md`).
-- `gen:skill-reference` — `gen:reference -- --out ~/.agents/skills/tttk-authoring/reference.md`.
+- `gen:skill-reference` — `gen:reference -- --out .agents/skills/tttk-authoring/reference.md`.
 
 ## 6. New skill file outlines
 
@@ -156,9 +160,10 @@ generator exists to prevent.
   checklist and a manual diff before deleting the old file.
 - **`known.ts` bloat.** Keep `details` to one paragraph per element; long-form
   material belongs in the companion files.
-- **Skill path portability.** `~` expansion in the generator; the skill lives
-  outside the repo, so the drift guard cannot cover it — document that
-  `gen:skill-reference` is the refresh command.
+- **Skill path portability.** The skill now lives in the repo, so
+  `gen:skill-reference` writes to a stable path; `~` expansion in `--out`
+  remains for ad-hoc targets. Activation is a symlink, which is environment
+  setup rather than repo state — documented in `docs/development.md`.
 - **Two references.** `docs/reference.md` and the skill copy are both generated
   from `known.ts`; they must be regenerated together. Note this in
   `docs/check-plan.md` §9.