fix: mask file-type bits from chmod, harden staging dir symlink check

- Add `import stat` to imports
- Use `stat.S_IMODE(mode)` before chmod in staging write (thread 20, line 1464)
- Use `stat.S_IMODE(preserved_mode)` and make chmod best-effort in
  `_restore_stranded_config_file` (thread 18, line 1492)
- Add `not rescue_staging_dir.is_symlink()` guard to cleanup (thread 19, line 1522)

Assisted-by: GitHub Copilot (model: claude-sonnet-4.6, autonomous)

Author: Copilot

src/specify_cli/extensions/__init__.py

@@ -14,6 +14,7 @@
 import os
 import re
 import shutil
+import stat
 import tempfile
 import zipfile
 from dataclasses import dataclass
@@ -1461,7 +1462,7 @@ def install_from_directory(
                     finally:
                         os.close(fd)
                     try:
-                        staged.chmod(mode)
+                        staged.chmod(stat.S_IMODE(mode))
                     except (NotImplementedError, OSError):
                         pass  # Best-effort; chmod may not be supported on all platforms.
             except Exception:
@@ -1489,7 +1490,10 @@ def _restore_stranded_config_file(
                 ) as tmp:
                     tmp_path = Path(tmp.name)
                     tmp.write(content)
-                tmp_path.chmod(preserved_mode)
+                try:
+                    tmp_path.chmod(stat.S_IMODE(preserved_mode))
+                except (NotImplementedError, OSError):
+                    pass  # Best-effort; chmod may not be supported on all platforms.
                 os.replace(tmp_path, target)
             except BaseException:
                 if tmp_path is not None and tmp_path.exists():
@@ -1518,7 +1522,7 @@ def _restore_stranded_config_file(
         # durable staging backup is no longer needed.  Raise on failure so
         # the install is not reported as successful while a stale backup
         # that could be misread on the next retry remains on disk.
-        if rescue_staging_dir.is_dir():
+        if rescue_staging_dir.is_dir() and not rescue_staging_dir.is_symlink():
             shutil.rmtree(rescue_staging_dir)
 
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