feat: add generated llms.txt skill index (#93)

Generate a root discovery catalog from public skill frontmatter and fail CI
when the committed index drifts. Add a fixture-based regression test for
bundle paths, nested-helper exclusion, normalized descriptions,
deterministic ordering, and stale-file recovery.

Closes #78

AI-assisted: yes (Jasper/Hermes Agent)

Co-authored-by: magnus919 <magnus919>

.github/workflows/validate.yml

@@ -26,6 +26,10 @@ jobs:
         run: ruby scripts/gen-claude-marketplace.rb
       - name: Validate Codex plugin packaging
         run: ruby scripts/gen-codex-plugin.rb
+      - name: Test llms.txt generator
+        run: ruby scripts/test-gen-llms-txt.rb
+      - name: Validate llms.txt catalog
+        run: ruby scripts/gen-llms-txt.rb
       - name: Build tracked Python packages
         run: |
           wheel_dir=$(mktemp -d)