fix(exec): isolate exec session managers

tests/agent/test_exec_session_isolation.py

@@ -0,0 +1,40 @@
+from types import SimpleNamespace
+from unittest.mock import MagicMock
+
+from nanobot.agent.loop import AgentLoop
+from nanobot.bus.queue import MessageBus
+
+
+def _provider() -> MagicMock:
+    provider = MagicMock()
+    provider.get_default_model.return_value = "test-model"
+    provider.generation = SimpleNamespace(max_tokens=4096, temperature=0.1, reasoning_effort=None)
+    return provider
+
+
+def test_agent_loops_do_not_share_exec_session_managers(tmp_path):
+    loop_a = AgentLoop(
+        bus=MessageBus(),
+        provider=_provider(),
+        workspace=tmp_path / "a",
+        model="test-model",
+        context_window_tokens=4096,
+    )
+    loop_b = AgentLoop(
+        bus=MessageBus(),
+        provider=_provider(),
+        workspace=tmp_path / "b",
+        model="test-model",
+        context_window_tokens=4096,
+    )
+
+    exec_a = loop_a.tools.get("exec")
+    stdin_a = loop_a.tools.get("write_stdin")
+    list_a = loop_a.tools.get("list_exec_sessions")
+    exec_b = loop_b.tools.get("exec")
+
+    assert exec_a._session_manager is loop_a._exec_session_manager
+    assert stdin_a._manager is loop_a._exec_session_manager
+    assert list_a._manager is loop_a._exec_session_manager
+    assert exec_b._session_manager is loop_b._exec_session_manager
+    assert loop_a._exec_session_manager is not loop_b._exec_session_manager