feat: download and install extra DMG/PKG/ZIP installers during restore

Adds a simple config file (~/.config/ssr/extra-installers.txt) where
users can list installer URLs for apps not available on Homebrew or MAS
(e.g. EasyMCP Desktop, internal tools).

lib/installers.sh:
  - Supports .dmg (mount → cp .app or run .pkg), .pkg, .zip
  - Skips entries already present in /Applications
  - Non-fatal: logs warnings and continues on failure

ssr sync:  copies extra-installers.txt to <backup_dir> alongside Brewfile
ssr restore: new `extra_installers` resumable step runs after shell_deps

Format: optional-name <url>   (# comments and blank lines ignored)
Example entry:
  EasyMCP https://github.com/Adobe-AIFoundations/easymcp/.../EasyMCP-Desktop-3.0.3-arm64.dmg

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

bin/ssr-sync

@@ -17,6 +17,8 @@ source "$SSR_ROOT/lib/apps.sh"
 source "$SSR_ROOT/lib/macos-prefs.sh"
 # shellcheck source=../lib/icloud.sh
 source "$SSR_ROOT/lib/icloud.sh"
+# shellcheck source=../lib/installers.sh
+source "$SSR_ROOT/lib/installers.sh"
 
 ssr::require_macos
 ssr::load_config
@@ -56,6 +58,9 @@ ssr::mackup::backup
 ssr::brew::dump "$backup_dir"
 ssr::brew::cleanup
 
+# Back up the extra-installers list alongside the Brewfile.
+ssr::installers::sync_list "$backup_dir"
+
 # Surface .app bundles that aren't covered by brew cask or the App Store —
 # they would otherwise be missed on `ssr restore` and need manual reinstall.
 if [[ "${SSR_SCAN_UNMANAGED_APPS:-true}" == "true" ]]; then