test(subagent): verify cascade exec termination on /stop

- terminate_by_owner kills matching sessions, skips others, handles
  empty owner case
- cancel_by_session calls terminate_by_owner on the session key

tests/agent/test_task_cancel.py

@@ -274,6 +274,27 @@ class TestSubagentCancellation:
         )
         assert await mgr.cancel_by_session("nonexistent") == 0
 
+    @pytest.mark.asyncio
+    async def test_cancel_by_session_terminates_exec_sessions(self):
+        from nanobot.agent.subagent import SubagentManager
+        from nanobot.agent.tools.exec_session import ExecSessionManager
+        from nanobot.bus.queue import MessageBus
+
+        bus = MessageBus()
+        mgr = SubagentManager(
+            workspace=MagicMock(),
+            bus=bus,
+            max_tool_result_chars=_MAX_TOOL_RESULT_CHARS,
+        )
+        # Replace the real exec session manager with a mock
+        mock_exec_mgr = AsyncMock(spec=ExecSessionManager)
+        mock_exec_mgr.terminate_by_owner = AsyncMock(return_value=0)
+        mgr._exec_session_manager = mock_exec_mgr
+
+        await mgr.cancel_by_session("test:c1")
+
+        mock_exec_mgr.terminate_by_owner.assert_awaited_once_with("test:c1")
+
     @pytest.mark.asyncio
     async def test_subagent_preserves_reasoning_fields_in_tool_turn(self, monkeypatch, tmp_path):
         from nanobot.agent.subagent import SubagentManager