feat(docs): add /docs/plugins catalog page fed by plugin-catalog/ extractor

- website/scripts/extract-plugins.py: reads plugin-catalog/*.yaml (+removed.yaml),
  emits static/api/plugins.json + plugins-meta.json; degrades to an empty
  catalog with exit 0 when plugin-catalog/ does not exist yet
- website/src/pages/plugins/: catalog page with search, tier tabs
  (All/Official/Community), capability chips, pinned-SHA repo links,
  copyable install commands, and an empty-state submission CTA
- cross-nav between Skills Hub and Plugin Catalog pages + navbar item
- user docs: user-guide/features/plugin-catalog.md (trust model, install,
  submission checklist, custom git-URL contrast), registered in sidebars.ts
- wired into deploy-site.yml and prebuild.mjs; artifacts gitignored
- tests: tests/website/test_extract_plugins.py

.github/workflows/deploy-site.yml

@@ -157,6 +157,9 @@ jobs:
       - name: Extract skill metadata for dashboard
         run: python3 website/scripts/extract-skills.py
 
+      - name: Extract plugin catalog for the Plugins page
+        run: python3 website/scripts/extract-plugins.py
+
       - name: Regenerate per-skill docs pages + catalogs
         run: python3 website/scripts/generate-skill-docs.py