New GitHub Actions check that scans every PR commit for
`Co-authored-by: ... claude ...` or `... anthropic ...` trailers and
fails the PR with a clear remediation message if found. Contributors
can still use AI tools; the trailer attribution must be removed before
the PR is eligible to merge, consistent with the project contributor
guidelines.
The workflow scans only commits introduced by the PR
(base.sha..head.sha), so existing history is untouched.
