feat(install): wire impeccable into the toolchain — deterministic design floor + /impeccable verbs

Complementary to frontend-design (kept: build-time aesthetic direction).
impeccable adds what the chain lacked: 45 deterministic anti-slop rules
(npx impeccable detect, exit 0/2, --json) — the design counterpart of the
semgrep gate — plus 23 design verbs under one /impeccable skill and
persistent per-project design context.

- plugins.lock.json: CLI pinned 3.2.0 (rules update = audit output change
  on unchanged code, LRN-077 class); skill dist = its own release track
- install-plugins.sh Step 8d: staged npx install (tmpdir) -> moved to
  skills-external/impeccable (machine-owned, gitignored, ctx7 pattern);
  never writes through the ~/.claude/skills symlink into the tracked tree
- update-all.sh: pin-honored refresh, Node<24 or failure -> dist kept
- Node >= 24 required (host at 22): steps skip gracefully, activation
  deferred to a deliberate Node bump
- link.sh EXTERNAL_SKILLS, profiles (design/web/web-full/full),
  plugin-advisor, CLAUDE.md design routing, design-gate, README, CHANGELOG
- NOT in design GATE-BLOCK yet: promotion after first dogfood

README.md

@@ -100,6 +100,7 @@ Versions are pinned in `plugins.lock.json`. To update: edit the file, then re-ru
 | `/code-clean` | Dead code removal, style/norm enforcement |
 | `/doc` | Documentation audit and sync — detect stale docs, patch |
 | `/seo` | Full SEO/GEO audit and optimization |
+| `/impeccable` | Design verbs (audit, polish, bolder…) + deterministic anti-slop detector (`npx impeccable detect`) |
 | `/commit-change` | Smart commit grouping from staged/unstaged changes |
 | `/gitflow` | Gitflow branch operations — bootstrap main+develop, start a typed branch, directed merge |
 | `/release-candidate` | Cut a versioned release — finalize version.txt + CHANGELOG, merge develop→main, tag, push |