Merge development into fix/co-author-trailer-after-merge-conflict

Development replaced the COMMIT_CO_AUTHOR_TRAILER constant with commit_co_author_trailer(), which adds the version and surface. The amend and the merge tests now call it. The "co-authored-by: sigit code" check still matches the versioned form. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Co-Authored-By: siGit Code <noreply@sigit.si>