fix(backup): handle files with pre-1980 timestamps

ZipFile.write() raises ValueError for files with mtime before 1980-01-01
(the ZIP format uses MS-DOS timestamps which can't represent earlier dates).
This crashes the entire backup. Add ValueError to the existing except clause
so these files are skipped and reported in the warnings summary, matching the
existing behavior for PermissionError and OSError.

hermes_cli/backup.py

@@ -201,7 +201,7 @@ def run_backup(args) -> None:
                 else:
                     zf.write(abs_path, arcname=str(rel_path))
                     total_bytes += abs_path.stat().st_size
-            except (PermissionError, OSError) as exc:
+            except (PermissionError, OSError, ValueError) as exc:
                 errors.append(f"  {rel_path}: {exc}")
                 continue