The merge handoff refused with head-branch-mismatch whenever the reused
worktree's HEAD wasn't on fusion/<id> (detached, recycled to main, or on
a sibling branch), even when the branch ref itself still held a clean,
task-attributed lineage. That wedged FN-5339-class tasks in review for
no good reason.
Add isBranchAuthoritativeForTask in branch-conflicts.ts (branch ref
exists, tip carries Fusion-Task-Id trailer, base..branch is foreign-
contamination-free) and use it in acquireReuseHandoff: when HEAD drifts
but the branch ref is authoritative, run a plain `git checkout <branch>`
inside the already-asserted-clean worktree, re-read HEAD, and emit a
branch:auto-reattach-authoritative audit. Refusal still fires unchanged
when the branch ref is missing, missing the trailer, or contaminated,
so FN-5363 strict-lease and foreign-commit guards remain authoritative.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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