When the worktree-recycle pool reassigned a path to a new task, the old
task's diff endpoints kept reading the new task's branch state — surfacing
unrelated commits as the original task's "files changed" list.
- Clear task.worktree/branch in the merger after the worktree is released
to the pool or removed, so the path no longer points anywhere.
- Validate the worktree's current branch matches task.branch in the three
worktree-backed diff endpoints; on mismatch return empty rather than
diffing against a foreign branch.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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