WorktreePool.prepareForTask now rejects empty/"HEAD" base values and verifies
that the worktree's HEAD actually landed at the resolved base SHA after
`git checkout --detach`. This closes the FN-5432 / FN-5255 contamination
pattern where a recycled worktree branched from a stale HEAD (reflog:
"branch: Created from HEAD") and pinned the new task's tip to the previous
occupant's commit.
SelfHealingManager.tryReanchorForeignOnlyContamination is invoked from both
PR-conflict and self-owned-branch-conflict catch sites before the
unrecoverable-pause path. When the conflicted branch carries only foreign
commits (no own work), the branch is reset to base via the existing
recoverForeignOnlyContamination flow instead of being escalated for human
adjudication.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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