feat(skill-manage): add 'lint' — library audit + promotion gate (composes skill-librarian)

[LeoLin990405]

What

Adds skill-manage.sh lint — the library-health check the forge → candidate → install path was missing — by composing with the standalone skill-librarian linter (not merging: grimoire stays the create/install side, skill-librarian stays the tiny vendor-neutral audit tool).

The linter is vendored as one file, scripts/lib/skill_lint.py (Python; stdlib + optional PyYAML, degrades to a regex parser when PyYAML is absent — consistent with the python3 already used in doctor).

Two modes

• skill-manage.sh lint [--index] — audit the canonical library: dup / near-dup / name-collision / dead [[wiki]] link / description-quality / stub / orphan. --index rewrites INDEX.md.
• skill-manage.sh lint --pack <dir> — promotion gate: check a candidate skill pack for name/content collisions against the library before installing it. Non-zero exit = do not promote.

Verified locally

lint --pack <colliding "pdf" candidate>   → ❌ 与库中已有 skill 同名: pdf   → exit 1
lint --pack <unique candidate>            → 0 error                        → exit 0
lint (audit ~/.claude/skills, 358 skills) → 0 error / 95 warn / 4 info      → exit 0

bash -n clean. No new hard deps.

Not a merge

skill-librarian keeps its own repo (broad, single-file, vendor-neutral); grimoire just vendors + calls it at the gate. Unix-compose, not one big熔炉.