feat: /stop cancels spawned subagents via session tracking

- 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

nanobot/agent/tools/spawn.py

@@ -15,11 +15,13 @@ class SpawnTool(Tool):
         self._manager = manager
         self._origin_channel = "cli"
         self._origin_chat_id = "direct"
+        self._session_key = "cli:direct"
     
     def set_context(self, channel: str, chat_id: str) -> None:
         """Set the origin context for subagent announcements."""
         self._origin_channel = channel
         self._origin_chat_id = chat_id
+        self._session_key = f"{channel}:{chat_id}"
     
     @property
     def name(self) -> str:
@@ -57,4 +59,5 @@ class SpawnTool(Tool):
             label=label,
             origin_channel=self._origin_channel,
             origin_chat_id=self._origin_chat_id,
+            session_key=self._session_key,
         )