feat(checkpoint): wire checkpoint tool into agent loop dispatch

tests/test_checkpoint_agent_dispatch.py

@@ -0,0 +1,16 @@
+"""Verify that checkpoint is routed through the agent loop, not the registry."""
+import json
+import pytest
+from model_tools import handle_function_call, _AGENT_LOOP_TOOLS
+
+
+def test_checkpoint_is_agent_loop_tool():
+    """checkpoint must be in _AGENT_LOOP_TOOLS so it gets agent-level state."""
+    assert "checkpoint" in _AGENT_LOOP_TOOLS
+
+
+def test_checkpoint_registry_dispatch_returns_error():
+    """Calling handle_function_call for checkpoint should return agent-loop error."""
+    result = handle_function_call("checkpoint", {"action": "read"})
+    data = json.loads(result)
+    assert "must be handled" in data["error"].lower()