The resume path in acquireTaskWorktree returned a reused worktree
without checking whether its branch contained foreign commits. If a
sibling task's tip had been baked into the branch at creation time,
the executor preflight would later fail contamination checks forever
(observed in the FN-5475 cascade).
The resume path now computes a fresh merge-base and runs
classifyBootstrapMisbinding. For the foreign-only / zero-own-commits
shape it re-anchors inline and emits a branch:reanchor audit event.
Mixed contamination continues to flow through the executor's
primary recovery path.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Squash-imported the working tree of fusion/fn-4410 as a single commit so this branch carries the dep's content without inheriting its individual commits. If the dep is later squash-merged to main, this commit's patch-id should match the merge and rebase cleanly.
Fusion-Task-Id: FN-4428
Fusion-Task-Lineage: ea174f50-49ac-4105-b6cf-db3bab0984fd
