fix: harden cron tool contract and repeat guard

tests/utils/test_runtime.py

@@ -0,0 +1,20 @@
+from nanobot.utils.runtime import repeated_tool_call_error, tool_call_signature
+
+
+def test_tool_call_signature_sorts_arguments_stably() -> None:
+    first = tool_call_signature("read_file", {"offset": 1, "path": "memory/history.jsonl"})
+    second = tool_call_signature("read_file", {"path": "memory/history.jsonl", "offset": 1})
+
+    assert first == second
+
+
+def test_repeated_tool_call_error_blocks_after_two_attempts() -> None:
+    seen: dict[str, int] = {}
+
+    assert repeated_tool_call_error("read_file", {"path": "a.txt"}, seen) is None
+    assert repeated_tool_call_error("read_file", {"path": "a.txt"}, seen) is None
+
+    error = repeated_tool_call_error("read_file", {"path": "a.txt"}, seen)
+
+    assert error is not None
+    assert "repeated identical call to 'read_file' blocked after 2 attempts" in error