Add public contribution rules and templates

[sam-bretz]

What changed

Extend the existing public privacy rules with publication guidance, add a Claude import, contribution and issue templates, a pull request checklist, and an automated check for common internal references.

This repository is in a different GitHub organization, so it does not inherit the new Metergraph organization defaults.

Verification

• Parsed issue and workflow YAML files.
• Ran the publication check against this branch; it passed.
• Confirmed the check rejects an internal reference in PR metadata.
• Reviewed the diff and ran git diff --check.

Admin follow-up

Private vulnerability reporting is disabled for this repository, though its existing security policy links to that route. An organization administrator needs to enable it. The issue form asks reporters not to post vulnerability details publicly.