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>

bin/ssr-sync

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# ssr-sync — back up Homebrew + Mackup state to the configured cloud dir.
+# ssr-sync — back up Homebrew + Mackup state to the configured cloud dir (copy-based mackup backup).
 
 set -euo pipefail
 SSR_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"