When a user sends a message while the agent is executing a task on the
gateway, the agent is now interrupted immediately — not silently queued.
Previously, messages were stored in _pending_messages with zero feedback
to the user, potentially leaving them waiting 1+ hours.
Root cause: Level 1 guard (base.py) intercepted all messages for active
sessions and returned with no response. Level 2 (gateway/run.py) which
calls agent.interrupt() was never reached.
Fix: Expand _handle_active_session_busy_message to handle the normal
(non-draining) case:
1. Call running_agent.interrupt(text) to abort in-flight tool calls
and exit the agent loop at the next check point
2. Store the message as pending so it becomes the next turn once the
interrupted run returns
3. Send a brief ack: 'Interrupting current task (10 min elapsed,
iteration 21/60, running: terminal). I'll respond shortly.'
4. Debounce acks to once per 30s to avoid spam on rapid messages
Reported by @Lonely__MH.
