Guard live Treehouse worktrees across spawn and reboot

[unhexquadium]

Reimplements a lost handoff branch from the accepted design.\n\n- Refuse fresh fm-spawn worktree leases that match any recorded local task worktree.\n- Re-register pid-less durable Treehouse lease guards for recorded local worktrees during bootstrap after reboot.\n- Cover both guard paths with behavioral tests.

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge, with no concrete blocking or independently actionable non-blocking issue identified.

The new guard paths fail closed for registration errors, serialize state updates with Treehouse’s advisory lock, preserve remote filesystem separation, and retain a post-allocation metadata collision check.

_{Reviews (1): Last reviewed commit: "no-mistakes(document): Document Treehous..." | Re-trigger Greptile}