fix(agents): preserve existing rules during symlink installation

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}")