fix(exec): clean up sessions on shutdown

tests/tools/test_exec_platform.py

@@ -114,6 +114,22 @@ class TestSpawnUnix:
         kwargs = mock_exec.call_args[1]
         assert kwargs["stdin"] == asyncio.subprocess.DEVNULL
 
+    @pytest.mark.asyncio
+    async def test_process_tree_starts_new_session(self):
+        with (
+            patch("nanobot.agent.tools.shell._IS_WINDOWS", False),
+            patch("asyncio.create_subprocess_exec", new_callable=AsyncMock) as mock_exec,
+        ):
+            mock_exec.return_value = AsyncMock()
+            await ExecTool._spawn(
+                "echo hi",
+                "/tmp",
+                {"HOME": "/tmp"},
+                process_tree=True,
+            )
+
+        assert mock_exec.call_args.kwargs["start_new_session"] is True
+
 
 class TestSpawnWindows: