chore(repo): stop tracking installer-managed files

Three paths are (re)generated by every install/update and should never be
committed:
- skills-external/frontend-design/ — install-plugins.sh Step 8b and
  update-all.sh cp the latest SKILL.md from the example-skills plugin cache
  over it, so it churned a diff each time Anthropic shipped an update. The
  source is always re-synced (example-skills is always installed), so no
  vendored copy is needed.
- .agents/ and skills-lock.json — `npx skills add` (darwin-skill) installs
  at project scope into the repo. Our own agents live in agents/ (no dot)
  and stay tracked; the dotted pollution dir is anchored-ignored (/.agents/).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UyNYwD4UccVw9ZCFZyJX55

.gitignore

@@ -119,3 +119,16 @@ desktop.ini
 # Profile cache — written by lib/profile.sh, read by hooks/statusline.sh
 .active-profile
 .gstack/
+
+# Frontend Design (Anthropic) — installed/refreshed from the example-skills
+# plugin cache by install-plugins.sh (Step 8b) and update-all.sh on every run.
+# Not vendored: tracking it produced a repo diff each time Anthropic shipped
+# an update. The source is always re-synced, so no offline copy is needed.
+skills-external/frontend-design/
+
+# npx `skills add` project-scope artifacts — darwin-skill copies itself into
+# the repo's .agents/ and writes skills-lock.json at root. Our own agents live
+# in agents/ (no dot) and stay tracked. Anchored to root so only the dotted
+# pollution dir is ignored.
+/.agents/
+/skills-lock.json