refine heartbeat session retention boundaries

tests/test_commands.py

@@ -477,6 +477,12 @@ def test_agent_hints_about_deprecated_memory_window(mock_agent_runtime, tmp_path
     assert "no longer used" in result.stdout
 
 
+def test_heartbeat_retains_recent_messages_by_default():
+    config = Config()
+
+    assert config.gateway.heartbeat.keep_recent_messages == 8
+
+
 def test_gateway_uses_workspace_from_config_by_default(monkeypatch, tmp_path: Path) -> None:
     config_file = tmp_path / "instance" / "config.json"
     config_file.parent.mkdir(parents=True)