check_contributors.py --identities ignores Co-authored-by trailers #1269

Where: scripts/check_contributors.py (git_authors).

Current gap: Only the commit author is read (%an|%ae). A person credited through a Co-authored-by: trailer, such as Voyagerroc-Code on commit 530b58d, never appears in the check, so a co-author with no entry or identity row is not reported.

Suggested approach: Also read the trailers (%(trailers:key=Co-authored-by,valueonly)), map their names and emails the same way and list the unmapped ones with the commit.

Done when: a test with a fake log containing a trailer reports an unmapped co-author and ignores a mapped one.