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

link.sh

@@ -71,7 +71,7 @@ if [ -d "$GSTACK_SRC/browse/dist" ]; then
   fi
 fi
 
-EXTERNAL_SKILLS=(emil-design-eng frontend-design design-motion-principles)
+EXTERNAL_SKILLS=(emil-design-eng frontend-design design-motion-principles impeccable)
 for _ext_skill in "${EXTERNAL_SKILLS[@]}"; do
   if [ -d "$REPO/skills-external/$_ext_skill" ]; then
     if [ -L "$CLAUDE/skills/$_ext_skill" ] && [ "$(readlink "$CLAUDE/skills/$_ext_skill")" = "$REPO/skills-external/$_ext_skill" ]; then