Why: review M5 - delegate_task spawned sub-agents outside the parent's Writable enforcement, the same unenforceability preflight already rejects for exec/MCP/host tools. Evidence: preflight rejection, delegation-still-works and legacy cases pass (verify-m5.json, verify-m5-host.json exit=0). Impact: preflight check only; empty Writable behavior unchanged. Refs: feature/audit-followups-pin-gate-m5