fix(subagent): cascade exec session termination on /stop

cancel_by_session() only cancelled asyncio tasks, leaving child
processes from exec sessions orphaned. Since each SubagentManager
now owns a dedicated ExecSessionManager, terminate those sessions
by owner_session_key after cancelling tasks.

Add ExecSessionManager.terminate_by_owner() to kill all sessions
for a given owner, and call it from cancel_by_session().

nanobot/agent/subagent.py

@@ -457,6 +457,7 @@ class SubagentManager:
             t.cancel()
         if tasks:
             await asyncio.gather(*tasks, return_exceptions=True)
+        await self._exec_session_manager.terminate_by_owner(session_key)
         return len(tasks)
 
     async def close(self) -> None:

nanobot/agent/tools/exec_session.py

@@ -334,6 +334,19 @@ class ExecSessionManager:
             )
         return len(sessions)
 
+    async def terminate_by_owner(self, owner_session_key: str) -> int:
+        """Terminate all sessions owned by owner_session_key. Returns count."""
+        async with self._lock:
+            victims = []
+            for sid, s in list(self._sessions.items()):
+                if s.owner_session_key == owner_session_key:
+                    victims.append(self._sessions.pop(sid))
+        await asyncio.gather(
+            *(s.kill() for s in victims),
+            return_exceptions=True,
+        )
+        return len(victims)
+
     async def _cleanup_locked(self) -> None:
         now = time.monotonic()
         stale = [