feat: add Codex plugin packaging (single-plugin, metadata-only) (#83)

* feat: add Codex plugin packaging (single-plugin, metadata-only)

Adds .codex-plugin/plugin.json with a skills array listing all public
skills, plus .agents/plugins/marketplace.json for one-command install:

  codex plugin marketplace add magnus919/agent-skills
  codex plugin install magnus919

Same pattern as mattpocock/skills — one plugin, explicit skill paths,
no dist/, no curation, no duplication. Bundle-internal helpers excluded
by the shared glob. CI check mode fails if the manifest drifts.

Closes #79

AI-assisted: yes (Jasper/Hermes Agent)

* chore: trigger CI

* chore: regenerate Codex plugin manifest to include neckbeard bundle

.github/workflows/validate.yml

@@ -24,6 +24,8 @@ jobs:
         run: python3 scripts/check-artifacts.py
       - name: Validate Claude Code marketplace
         run: ruby scripts/gen-claude-marketplace.rb
+      - name: Validate Codex plugin packaging
+        run: ruby scripts/gen-codex-plugin.rb
       - name: Build tracked Python packages
         run: |
           wheel_dir=$(mktemp -d)