chore(design-gate): repoint agents + test to profile remedy

The gate's remedy is now /profile design (not an atomic ui-ux-pro-max toggle),
but several callers still described the old "ask user to activate ui-ux-pro-max"
behavior. Repoint the prose — the `Follow lib/design-gate.md` pointers stayed
valid (the gate was extended in place), so only descriptions change:

- agents/feater.md, hotfixer.md, bugfixer.md (STEP 0.5/1.5 gate bullet):
  signals found → run design-tool-gate.sh; if INCOMPLETE → /profile design.
- agents/feater.md rule 6: trigger is "design toolchain incomplete", not
  "ui-ux-pro-max inactive".
- agents/plugin-advisor.md: gate points at /profile design when the toolchain
  is incomplete.
- skills/feat/test-prompts.json (id 2): expected behavior updated; still valid JSON.

No logic change — descriptions only, aligned with the profile-based gate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

skills/feat/test-prompts.json

@@ -1,5 +1,5 @@
 [
   {"id": 1, "prompt": "Add a /health endpoint to the API that returns 200 OK with version info", "expected": "Light planning, direct implementation in 1-3 files, no design brainstorm, no plugin check, run build/tests after"},
-  {"id": 2, "prompt": "Add dark mode toggle to the settings page", "expected": "Detect UI signal → trigger design gate (offer ui-ux-pro-max if inactive), then implement small feature"},
+  {"id": 2, "prompt": "Add dark mode toggle to the settings page", "expected": "Detect UI signal → trigger design gate (run design-tool-gate.sh; if toolchain incomplete, point at /profile design), then implement small feature"},
   {"id": 3, "prompt": "Petite feature : afficher un badge 'New' à côté des items créés cette semaine", "expected": "FR trigger, scope check (≤5 files), implement directly, no orchestrator overhead"}
 ]