fix: require anchored structured file edits

tests/test_agent.py

@@ -24,6 +24,8 @@ def test_system_prompt_requires_precise_diff_hygiene() -> None:
     assert "Never create backup files" in _SYSTEM_TEMPLATE
     assert "failure-report transport code" in _SYSTEM_TEMPLATE
     assert "Preserve unrelated lines exactly" in _SYSTEM_TEMPLATE
+    assert "small localized edits around a uniquely matching existing anchor" in _SYSTEM_TEMPLATE
+    assert "Never reconstruct or rewrite an entire structured file" in _SYSTEM_TEMPLATE
     assert "closest existing test class" in _SYSTEM_TEMPLATE
     assert "inspect all key columns and cardinality" in _SYSTEM_TEMPLATE
     assert "remove trailing spaces" in _SYSTEM_TEMPLATE