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/mackup.sh

@@ -238,7 +238,8 @@ ssr::mackup::generate_custom_cfg() {
         return 1
     }
     # Return true when <path> is already present in the Mackup cloud backup.
-    _in_backup() { [[ -e "$mackup_dir/$1" ]]; }
+    # SSR seeds ~/.mackup.cfg with directory=mackup — data lives under that subdir.
+    _in_backup() { [[ -e "$mackup_dir/mackup/$1" ]]; }
 
     local -a items=()
 
@@ -288,23 +289,21 @@ ssr::mackup::ensure_installed() {
 }
 
 # Writes ~/.mackup.cfg pointing the engine at the cloud directory.
-# Idempotent: rewrites only if content differs.
+# Never overwrites an existing different file without confirmation: shows a
+# unified diff, then prompts (TTY) or requires SSR_ALLOW_CONF_OVERWRITE=yes.
 ssr::mackup::configure() {
     local mackup_dir="$1"
     local cfg="$HOME/.mackup.cfg"
-    local desired
+    local desired tmp
     desired="$(printf '[storage]\nengine = file_system\npath = %s\ndirectory = mackup\n' "$mackup_dir")"
-
-    if [[ -f "$cfg" ]] && [[ "$(cat "$cfg")" == "$desired" ]]; then
+    tmp="$(mktemp "${TMPDIR:-/tmp}/ssr-mackup-cfg.XXXXXX")"
+    printf '%s\n' "$desired" > "$tmp"
+    if ssr::replace_file_with_confirm "$cfg" "$tmp"; then
+        rm -f "$tmp"
         return 0
     fi
-
-    if [[ -L "$cfg" || -f "$cfg" ]]; then
-        cp -f "$cfg" "$cfg.bak.$(date +%s)" 2>/dev/null || true
-        rm -f "$cfg"
-    fi
-    printf '%s\n' "$desired" > "$cfg"
-    ssr::ok "Wrote $cfg"
+    rm -f "$tmp"
+    return 1
 }
 
 # Force iCloud Drive to materialize placeholder (.icloud) files under the