The #45966 cross-process coherence guard popped the stale cached agent
and then called the blocking _cleanup_agent_resources (memory-provider
shutdown, tool-resource teardown, async-client teardown) while still
holding _agent_cache_lock, on the gateway event-loop thread. While that
ran, _sweep_idle_cached_agents (driven by _session_expiry_watcher)
blocked acquiring the same lock and the asyncio loop stalled for minutes,
tripping repeated Discord 'heartbeat blocked' warnings.
Fix mirrors the cap-enforcer / idle-sweep paths: pop the stale entry
under the lock, release it, then schedule the SOFT release on a daemon
thread. The soft path (_release_evicted_agent_soft) is also more correct
here than the hard teardown the regression used — the same session
rebuilds a fresh agent immediately after invalidation, so its terminal
sandbox / browser / bg processes (keyed on task_id) must be preserved
for the rebuilt agent to inherit, not torn down.
Verified the cross-process site was the only cleanup-under-lock instance;
the other _cleanup_agent_resources call sites run outside the lock.
811df74a10