Audit of all shared-member merge + self-healing finalize paths: routing,
merger finalize-success, and the 6 self-healing recovery paths were already
group-branch-safe (FN-5846). Found a residual of the 2026-05-23 lost-work
incident bug #2: already-merged-detector's ancestry strategy used bare
git log --grep first-hit, and the ownership regex made the conventional scope
optional (bare 'feat:' matched). Anchor attribution on trailers or task-scoped
subject; scan candidates instead of accepting the first grep hit. Adds real-git
characterization tests.
cad44b1f56