dropAutostashBySha resolved SHA→stash@{N} then ran git stash drop ${ref}
non-atomically. Any other process (interactive shell, parallel merger,
fix-agent) pushing a stash between resolve and drop shifted the index, so
we silently dropped the wrong entry while leaving ours behind. The task
log then claimed "Restored pre-merge autostash X cleanly" even though the
stash was still in the list — observed on FN-3558 (e81e922) and others.
Verify the ref still resolves to our SHA via git rev-parse before dropping;
on mismatch, re-resolve and retry up to 5x. Return success/failure so the
caller can record honest status to the task feed instead of unconditionally
logging "cleanly".
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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