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-sync

@@ -48,12 +48,6 @@ ssr::brew::upgrade
 # Timeout (seconds) is configurable; default 180s.
 ssr::icloud::wait "$mackup_dir" "${SSR_ICLOUD_WAIT_TIMEOUT:-180}"
 
-# Auto-generate ~/.mackup/ssr-custom.cfg for dot-dirs not yet covered by
-# Mackup's built-in definitions (e.g. .claude, .cursor, .easymcp, …).
-if [[ "${SSR_MACKUP_AUTODISCOVER:-true}" == "true" ]]; then
-    ssr::mackup::generate_custom_cfg "$mackup_dir"
-fi
-
 ssr::mackup::backup
 ssr::brew::dump "$backup_dir"
 ssr::brew::cleanup