Adds /queue <prompt> (alias /q) that queues a message for the next
turn while the agent is busy, without interrupting the current run.
- CLI: /queue <prompt> puts it in _pending_input for the next turn
- Gateway: /queue <prompt> creates a pending MessageEvent on the
adapter, picked up after the current agent run finishes
- Enter still interrupts as usual (no behavior change)
- /queue with no prompt shows usage
- /queue when agent is idle tells user to just type normally
Co-authored-by: Test <test@test.com>