feat(skills): centralized skills index — eliminate GitHub API calls for search/install

Add a CI-built skills index served from the docs site. The index is
crawled daily by GitHub Actions, resolves all GitHub paths upfront, and
is cached locally by the client. When the index is available:

- Search uses the cached index (0 GitHub API calls, was 23+)
- Install uses resolved paths from index (6 API calls for file
  downloads only, was 31-45 for discovery + downloads)

Total: 68 → 6 GitHub API calls for a typical search + install flow.
Unauthenticated users (60 req/hr) can now search and install without
hitting rate limits.

Components:
- scripts/build_skills_index.py: Crawl all sources (skills.sh, GitHub
  taps, official, clawhub, lobehub), batch-resolve GitHub paths via
  tree API, output JSON index
- tools/skills_hub.py: HermesIndexSource class — search/fetch/inspect
  backed by the index, with lazy GitHubSource for file downloads
- parallel_search_sources() skips external API sources when index is
  available (0 GitHub calls for search)
- .github/workflows/skills-index.yml: twice-daily CI build + deploy
- .github/workflows/deploy-site.yml: also builds index during docs deploy

Graceful degradation: when the index is unavailable (first run, network
down, stale), all methods return empty/None and downstream sources
handle the request via direct API as before.

.github/workflows/deploy-site.yml

@@ -41,11 +41,19 @@ jobs:
           python-version: '3.11'
 
       - name: Install PyYAML for skill extraction
-        run: pip install pyyaml
+        run: pip install pyyaml httpx
 
       - name: Extract skill metadata for dashboard
         run: python3 website/scripts/extract-skills.py
 
+      - name: Build skills index (if not already present)
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          if [ ! -f website/static/api/skills-index.json ]; then
+            python3 scripts/build_skills_index.py || echo "Skills index build failed (non-fatal)"
+          fi
+
       - name: Install dependencies
         run: npm ci
         working-directory: website