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.

lib/cloud.sh

@@ -47,12 +47,14 @@ ssr::cloud::backup_dir() {
     printf '%s/%s/%s - %s\n' "$cloud" "$subdir" "$host" "$uuid"
 }
 
-# Mackup data lives at the cloud root in a fixed folder. Mackup itself
-# resolves its engine via ~/.mackup.cfg which the install routine seeds.
+# Parent directory for Mackup file_system storage: same cloud subfolder as
+# per-host backups (SSR_BACKUP_SUBDIR). ~/.mackup.cfg sets path=<this> and
+# directory=mackup — Mackup places files under <this>/mackup/ (not SSR-controlled).
 ssr::cloud::mackup_dir() {
-    local cloud
+    local cloud subdir
     cloud="$(ssr::cloud::resolve_dir)"
-    printf '%s/%s\n' "$cloud" "${SSR_MACKUP_SUBDIR:-Mackup}"
+    subdir="${SSR_BACKUP_SUBDIR:-BACKUP}"
+    printf '%s/%s\n' "$cloud" "$subdir"
 }
 
 ssr::cloud::ensure_available() {