The previous regex matched any line containing both 'co-authored-by'
and 'claude'/'anthropic' anywhere on the line, which flagged prose
*about* the trailer, not just the trailer itself. This caught the
commit that introduced the lint (subject: "ops: add CI lint for
Co-Authored-By/Claude/Anthropic trailers") as a false positive.
Anchor the match to `^[[:space:]]*co-authored-by:` so it only fires
on an actual trailer line (Key: value at line start), not a mention
in a commit subject or body. Add a --self-test mode with fixtures
covering three genuine-trailer spellings, an ordinary message, and
the PR's own subject line as the prose-mention regression case.
Refs #4432
9e618b3b90