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.bash

@@ -13,7 +13,7 @@ _ssr() {
         cword=$COMP_CWORD
     fi
 
-    local commands="install sync restore update schedule config completions status version help"
+    local commands="install sync restore update schedule config completions status mackup version help"
     local global_flags="-c --config -h --help"
 
     # Detect command (first non-flag word after `ssr`).
@@ -60,6 +60,9 @@ _ssr() {
         completions)
             COMPREPLY=( $(compgen -W "install uninstall status" -- "$cur") )
             ;;
+        mackup)
+            COMPREPLY=( $(compgen -W "scan list clean" -- "$cur") )
+            ;;
         *)
             COMPREPLY=()
             ;;