Contamination on fusion/<id> branches (FN-5233 was the recent example:
two untrailered feat(FN-5353): commits sitting on fusion/fn-5233) used
to be invisible until merge time, days after it happened. The executor
already runs assertCleanBranchAtBase at worktree acquisition and reclaim
— the gap was the active session window itself.
Add reportBranchAttribution(repoDir, branch, baseSha, taskId) which
walks base..branch and bins every commit into ownTrailed (healthy),
ownUntrailed (subject tag but commit-msg hook didn't fire), foreign
(different FN-id), or unattributed (no subject pattern, no trailer —
typically a hand-merge or plumbing commit). Wire it into the executor
right after captureModifiedFiles in the post-session path: when any
anomaly bucket is non-empty, emit a structured branch:attribution-
anomaly audit event and a task log entry. The audit itself is wrapped
in a try/catch so a probe failure never destabilizes a completing
session. New branch:attribution-anomaly and branch:auto-reattach-
authoritative GitMutationType variants accept the structured metadata
(the latter for the handoff re-attach added earlier this session).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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