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.

bin/ssr-sync

@@ -39,7 +39,8 @@ ssr::icloud::prefetch "$mackup_dir"
 
 ssr::brew::ensure_installed
 ssr::mackup::ensure_installed
-ssr::mackup::configure "$mackup_dir"
+_mackup_cfg_ok=true
+ssr::mackup::configure "$mackup_dir" || _mackup_cfg_ok=false
 
 ssr::brew::update
 ssr::brew::upgrade
@@ -48,7 +49,11 @@ ssr::brew::upgrade
 # Timeout (seconds) is configurable; default 180s.
 ssr::icloud::wait "$mackup_dir" "${SSR_ICLOUD_WAIT_TIMEOUT:-180}"
 
-ssr::mackup::backup
+if [[ "$_mackup_cfg_ok" == true ]]; then
+    ssr::mackup::backup
+else
+    ssr::warn "Skipping mackup backup — ~/.mackup.cfg was not updated (see messages above)."
+fi
 ssr::brew::dump "$backup_dir"
 ssr::brew::cleanup