#35994 moved /new reset cleanup off the loop, but _cleanup_agent_resources
(agent.close() subprocess teardown; shutdown_memory_provider() plugin IO) was
still called INLINE on the event loop from three other sites:
- _session_expiry_watcher (5-min idle sweep) — live loop
- _handle_message_with_agent cache-hygiene re-eviction — live loop
- _finalize_shutdown_agents / stop() idle-cache loop — shutdown
A wedged memory provider on any of these froze the loop: bot goes silent,
runtime-status updated_at heartbeat stops advancing, and SIGTERM can't be
serviced (requires kill -9) — exactly the #53175 zombie pattern.
Adds _cleanup_agent_resources_off_loop: a bounded (30s) worker-thread offload
mirroring the #35994 reset fix, and routes all four sites through it.
