Replace the "ahead-of" heuristic with a structural check that compares
the HEAD↔origin/$DEFAULT_BRANCH merge-base to origin/$DEFAULT_BRANCH
itself. The previous count-based warning fired on legitimate WIP that
was simply ahead of the default branch — the correct signal is that
the branch did not fork from the default branch in the first place.
Addresses CodeRabbit review on PR #2921.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>