fix(agents): preserve existing rules during symlink installation

agents/README.md

@@ -39,7 +39,8 @@ Behavior:
 
 - Each target is handled independently.
 - Missing destination directories are skipped.
-- Existing destination file/symlink is replaced with a symlink to this repo source file.
+- Existing regular files are preserved as `.bak` backups (numbered when a backup already exists).
+- Destination links are replaced atomically; a failed link creation leaves the destination intact.
 
 The installer links only `AGENTS.md`; it does not install permission configuration, skills, or CLIs.
 The repository-root `AGENTS.md` contains guidance for this Nix configuration repository. It is not

agents/install-rules.py

@@ -2,6 +2,7 @@
 
 import os
 import sys
+import tempfile
 from pathlib import Path
 
 
@@ -12,10 +13,24 @@ def install_one(target_dir: Path, source_file: Path, target_name: str) -> None:
 
     target_file = target_dir / target_name
 
-    if target_file.exists() or target_file.is_symlink():
-        target_file.unlink()
+    if target_file.is_file() and not target_file.is_symlink():
+        # Preserve the old inode without reading its contents or replacing an earlier backup.
+        index = 0
+        while True:
+            suffix = ".bak" if index == 0 else f".bak.{index}"
+            backup = target_file.with_name(target_file.name + suffix)
+            try:
+                os.link(target_file, backup)
+                break
+            except FileExistsError:
+                index += 1
+        print(f"backed up  {target_file} -> {backup}")
 
-    target_file.symlink_to(source_file)
+    # Create the link on the same filesystem before atomically replacing the destination.
+    with tempfile.TemporaryDirectory(prefix=".install-rules-", dir=target_dir) as staging:
+        link = Path(staging) / target_name
+        link.symlink_to(source_file)
+        link.replace(target_file)
     print(f"linked  {target_file} -> {source_file}")
 
 

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()