fix(mackup): update backup path and enhance configuration handling

- Corrected the backup path in the `_in_backup` function to reflect the new structure under `mackup/`.
- Improved error handling in `ssr::mackup::configure` to skip restore if the configuration file is not updated, providing user feedback.
- Updated `ssr::sync` to conditionally execute backup based on configuration validity, enhancing robustness.
- Revised documentation in configuration examples to clarify the new Mackup directory structure and its implications for users.

config/post-restore.sh.example

@@ -13,7 +13,7 @@
 # Available variables:
 #   SSR_ROOT       — path to the ssr repository
 #   backup_dir     — cloud backup directory for this host
-#   mackup_dir     — cloud Mackup directory
+#   mackup_dir     — cloud path parent for Mackup (~/.mackup.cfg path=…; files under …/mackup/)
 #
 # EXAMPLES
 # -------------------------------------------------------------------------