Two fixes from PR review:
1. Session expiry was looking in _running_agents for the cached agent,
but idle expired sessions live in _agent_cache. Now checks
_agent_cache first, falls back to _running_agents.
2. Global cleanup in stop() was missing process_registry.kill_all(),
so background processes from agents evicted without close() (branch,
fallback) survived shutdown.
9555a0cf31