fix: tighten agent patch hygiene

tests/test_agent.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from lai_automation.agent import MiniSWEAgentRunner
+from lai_automation.agent import _SYSTEM_TEMPLATE, MiniSWEAgentRunner
 from lai_automation.models import Issue
 
 
@@ -18,3 +18,10 @@ def test_task_uses_issue_text_not_github_parser() -> None:
     assert "Issue #42: Wrong result" in task
     assert issue.html_url in task
     assert issue.body in task
+
+
+def test_system_prompt_requires_precise_diff_hygiene() -> None:
+    assert "Never create backup files" in _SYSTEM_TEMPLATE
+    assert "Preserve unrelated lines exactly" in _SYSTEM_TEMPLATE
+    assert "inspect all key columns and cardinality" in _SYSTEM_TEMPLATE
+    assert "remove trailing spaces" in _SYSTEM_TEMPLATE