fix: enforce attribution messages

[nothingnesses]

Summary

• Apply one structural attribution policy to reachable commit messages and pull-request metadata.
• Parse the GitHub event file without evaluating untrusted title or body text.
• Re-run the required workflow after pull-request metadata edits.
• Cover Claude, GPT or Codex, Gemini, Copilot, safe prose, malformed events, and existing history rules with 43 scratch cases.

Review

Independent review found a versioned GPT model-name gap and a carriage-return-only hardening case. Separate triage confirmed only the GPT gap under the accepted criteria. One scoped fix added direct commit and pull-request tests, and focused verification passed.

Verification

• just ci
• 429 unit tests and all integration tests
• 43 attribution regression cases
• Complete reachable-history attribution check