feat: add Claude Code plugin marketplace (metadata-only catalog) (#80)

Adds .claude-plugin/marketplace.json exposing all 95 public skills as
installable plugins via /plugin marketplace add magnus919/agent-skills.

Metadata-only approach: each entry uses source './' + skills ['./<name>']
+ strict:false, so no per-skill plugin.json or directory restructuring is
needed. Bundle-internal helper skills are excluded; bundle entrypoints are
included.

- scripts/gen-claude-marketplace.rb: generates and validates the manifest
- CI step fails if marketplace.json drifts from the skill tree
- README: Claude Code install instructions

Closes #76

AI-assisted: yes (Jasper/Hermes Agent)

.github/workflows/validate.yml

@@ -22,6 +22,8 @@ jobs:
         run: ruby scripts/validate-skills.rb
       - name: Check tracked repository artifacts
         run: python3 scripts/check-artifacts.py
+      - name: Validate Claude Code marketplace
+        run: ruby scripts/gen-claude-marketplace.rb
       - name: Build tracked Python packages
         run: |
           wheel_dir=$(mktemp -d)