refactor(mackup): transition to copy-based backup and restore

- Updated `ssr sync` and `ssr restore` commands to use `mackup backup` and `mackup restore`, replacing the previous symlink-based approach.
- Added guidance for users with legacy symlink setups to run `ssr mackup unlink` for recovery.
- Revised documentation in README, completions, and configuration examples to reflect the new copy-based methodology for improved reliability on newer macOS versions.
- Enhanced comments in the codebase to clarify the changes and their implications for users.

Co-authored-by: Cursor <cursoragent@cursor.com>

completions/_ssr

@@ -65,7 +65,7 @@ _ssr() {
                         'scan[Suggest and confirm ssr-*.cfg files]' \
                         'list[List generated ssr-*.cfg files]' \
                         'clean[Remove all generated ssr-*.cfg files]' \
-                        'unlink[mackup link uninstall — revert symlinks]'
+                        'unlink[undo symlink Mackup — legacy recovery]'
                     ;;
             esac
             ;;