When the squash commit was built off a stale integration tip, the FF guard
in advanceIntegrationBranchRef refused the swap with reason
`non-fast-forward-advance` — but the caller only mapped `concurrent-advance`
to IntegrationBranchConcurrentAdvanceError, so the non-FF case fell through
as a plain Error and failed the task. Both reasons share a root cause
(integration moved during the merge window), so they now share the
FN-4500/FN-5083 rebind/retry path.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>