Enforce publish metadata guardrails

[TacoRocket]

What changed

• added a shared publish-metadata validator for branch names and PR titles
• updated the local pre-push hook to use the shared branch-name check
• added a CI metadata-policy job so pull requests with forbidden branding in the title are blocked server-side
• added focused tests and refreshed local guardrail docs

Why

The repo already blocked codex branch names locally, but PR titles were still only a written convention in agents.md. This makes publish metadata enforcement explicit so the same mistake cannot slip through again.

Impact

Publish flow now rejects forbidden branch names locally and rejects Codex-branded PR titles in CI.

Validation

• python3 -m pytest tests/test_publish_metadata.py -q
• python3 -m ruff check scripts/validate_publish_metadata.py tests/test_publish_metadata.py
• bash -n .githooks/pre-push scripts/setup_local_guardrails.sh
• YAML parse check for .github/workflows/ci.yml
• git push -u origin review/publish-metadata-guardrails pre-push guardrails: passed