feat: ssr mackup scan|list|clean — on-demand cfg generation with confirmation

Instead of silently writing ~/.mackup/ssr-*.cfg on every sync, generation
is now an explicit opt-in command: ssr mackup scan

  ssr mackup scan   — dry-run: shows NEW/UPD/DEL changes, prompts before writing
  ssr mackup list   — list existing generated files
  ssr mackup clean  — remove all generated files (with confirmation)

ssr sync no longer calls generate_custom_cfg automatically.
Completions updated for zsh, bash, fish.

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

completions/_ssr

@@ -15,6 +15,7 @@ _ssr() {
         'config:Show or edit the active config file'
         'completions:Install or remove shell completions'
         'status:Show last sync time, cloud target, schedule state'
+        'mackup:Manage auto-generated Mackup app definitions'
         'version:Print version'
         'help:Show top-level help'
     )
@@ -59,6 +60,12 @@ _ssr() {
                         'uninstall[Remove shell completions]' \
                         'status[Show installed completions]'
                     ;;
+                mackup)
+                    _values 'mackup action' \
+                        'scan[Suggest and confirm ssr-*.cfg files]' \
+                        'list[List generated ssr-*.cfg files]' \
+                        'clean[Remove all generated ssr-*.cfg files]'
+                    ;;
             esac
             ;;
     esac