fix(mackup): safer defaults + recovery after broken symlink sync

- Default-off SSR_MACKUP_PRESYMLINK for DynamicProfiles pre-symlink (was
  risking empty/wrong iCloud trees and breaking iTerm2).
- Remove incorrect skip entries: built-in cfgs use different paths than
  ~/.zsh, ~/.vscode*, ~/.iterm2 — skipping hid real overlap from scan only
  but the presymlink change was the main footgun.
- Add `ssr mackup unlink` → `mackup -f link uninstall` to copy configs back
  from Mackup storage and drop symlinks (official undo).
- Docs: ssr.conf.example, README sync/restore wording, completions.

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

completions/ssr.bash

@@ -61,7 +61,7 @@ _ssr() {
             COMPREPLY=( $(compgen -W "install uninstall status" -- "$cur") )
             ;;
         mackup)
-            COMPREPLY=( $(compgen -W "scan list clean" -- "$cur") )
+            COMPREPLY=( $(compgen -W "scan list clean unlink" -- "$cur") )
             ;;
         *)
             COMPREPLY=()