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

@@ -64,7 +64,8 @@ _ssr() {
                     _values 'mackup action' \
                         'scan[Suggest and confirm ssr-*.cfg files]' \
                         'list[List generated ssr-*.cfg files]' \
-                        'clean[Remove all generated ssr-*.cfg files]'
+                        'clean[Remove all generated ssr-*.cfg files]' \
+                        'unlink[mackup link uninstall — revert symlinks]'
                     ;;
             esac
             ;;