fix(agents): preserve existing rules during symlink installation

agents/test_install_rules.py

@@ -0,0 +1,46 @@
+import importlib.util
+import tempfile
+import unittest
+from pathlib import Path
+from unittest.mock import patch
+
+
+spec = importlib.util.spec_from_file_location(
+    "install_rules", Path(__file__).with_name("install-rules.py")
+)
+installer = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(installer)
+
+
+class InstallRulesTests(unittest.TestCase):
+    def test_existing_file_is_backed_up_without_overwriting_backup(self):
+        with tempfile.TemporaryDirectory() as directory:
+            root = Path(directory)
+            source = root / "source.md"
+            source.write_text("global rules")
+            target = root / "AGENTS.md"
+            target.write_text("personal rules")
+            backup = root / "AGENTS.md.bak"
+            backup.write_text("older rules")
+            installer.install_one(root, source, target.name)
+            self.assertEqual(target.resolve(), source)
+            self.assertEqual(backup.read_text(), "older rules")
+            self.assertEqual((root / "AGENTS.md.bak.1").read_text(), "personal rules")
+
+    def test_failed_link_creation_preserves_existing_file(self):
+        with tempfile.TemporaryDirectory() as directory:
+            root = Path(directory)
+            source = root / "source.md"
+            source.write_text("global rules")
+            target = root / "AGENTS.md"
+            target.write_text("personal rules")
+            with patch.object(Path, "symlink_to", side_effect=OSError("link failed")):
+                with self.assertRaises(OSError):
+                    installer.install_one(root, source, target.name)
+            self.assertTrue(target.is_file())
+            self.assertFalse(target.is_symlink())
+            self.assertEqual(target.read_text(), "personal rules")
+
+
+if __name__ == "__main__":
+    unittest.main()