- SubagentManager tracks _session_tasks: session_key -> {task_id, ...}
- cancel_by_session() cancels all subagents for a session
- SpawnTool passes session_key through to SubagentManager
- /stop response reports subagent cancellation count
- Cleanup callback removes from both _running_tasks and _session_tasks
Builds on #1179
- Add commands.py with CommandDef registry, parse_command(), get_help_text()
- Refactor run() to dispatch messages as asyncio tasks (non-blocking)
- /stop is an 'immediate' command: handled inline, cancels active task
- Global processing lock serializes message handling (safe for shared state)
- _pending_tasks set prevents GC of dispatched tasks before lock acquisition
- _dispatch() registers/clears active tasks, catches CancelledError gracefully
- /help now auto-generated from COMMANDS registry
Closes#849
