Add public metadata guard

[Will-thom]

Summary

• Add a required CI guard for public PR metadata
• Block branch names, PR titles, and commit messages that mention blocked coding tools
• Include the guard in All CI checks
• Document the policy in contributor, CI, guardrail, permission, and PR checklist docs

Validation

• git diff --check
• ./.github/scripts/check-public-metadata.sh with clean metadata through Git Bash
• negative local check with a blocked PR title

Notes

After this PR merges, the repository ruleset layer should be added on main for commit-message enforcement.

[github-actions]

Hi @Will-thom, thank you for contributing to PR Maven CLI.

I appreciate the time you spent opening this pull request. This project is intentionally shaped for focused, reviewable open source contributions, so your input helps the tool become more useful for Java and Maven teams.

A maintainer will review the context, labels, and next steps as soon as possible. If this is your first contribution here, welcome aboard.

Useful links:

• Contributing guide: CONTRIBUTING.md
• Testing guide: docs/testing.md
• CI/CD guide: docs/ci.md

Thank you for helping build a deterministic, local-first Maven failure triage tool.