When mergeIntegrationWorktree=reuse-task-worktree but the task has no
worktree or the worktree classifies as unusable, the merger now:
1. Creates a fresh worktree at the standard path with the fusion/<id> branch
2. Installs identity-guard hooks
3. Updates task.worktree + task.branch in the store
4. Emits merge:reuse-worktree-fresh-acquire / merge:reuse-worktree-fresh-acquired
5. Retries the handoff from the new worktree
Only falls back to cwd-main as last resort when fresh acquisition also
fails. Lease/liveness conflicts (worktree exists and classifies OK but
handoff refused on active-session or lease grounds) are re-thrown — not
masked with cwd-main.
New audit event types: merge:reuse-worktree-fresh-acquire,
merge:reuse-worktree-fresh-acquired
a03a5de235