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>

bin/ssr

@@ -34,6 +34,7 @@ Commands:
   config show|edit     Print or edit the active config file.
   completions          Install or remove shell completions (zsh/bash/fish).
   status               Show last sync time, cloud target, and scheduled state.
+  mackup scan|list|clean  Manage auto-generated Mackup app definitions.
   version              Print version.
 
 Options:
@@ -62,7 +63,7 @@ done
 
 cmd="$1"; shift
 case "$cmd" in
-    install|sync|restore|update|schedule|config|completions|status)
+    install|sync|restore|update|schedule|config|completions|status|mackup)
         target="$SSR_BIN_DIR/ssr-$cmd"
         [[ -x "$target" ]] || ssr::die "Subcommand not executable: $target"
         exec "$target" "$@"