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-restore

@@ -25,6 +25,8 @@ source "$SSR_ROOT/lib/restore-state.sh"
 source "$SSR_ROOT/lib/icloud.sh"
 # shellcheck source=../lib/shell.sh
 source "$SSR_ROOT/lib/shell.sh"
+# shellcheck source=../lib/installers.sh
+source "$SSR_ROOT/lib/installers.sh"
 
 ssr::require_macos
 ssr::load_config
@@ -110,6 +112,16 @@ else
     ssr::rs::set "step_shell_deps" "skipped"
 fi
 
+# Extra installers: DMG / PKG / ZIP not available via Homebrew or MAS.
+# List file is backed up to <backup_dir>/extra-installers.txt by `ssr sync`.
+_extra_installers_file="$backup_dir/extra-installers.txt"
+if [[ -f "$_extra_installers_file" ]]; then
+    ssr::rs::run extra_installers "Extra installers" \
+        ssr::installers::install_from_file "$_extra_installers_file"
+else
+    ssr::rs::set "step_extra_installers" "skipped"
+fi
+
 ssr::rs::run macos_defaults  "macOS defaults"          ssr::macos::apply_defaults
 
 if [[ "${SSR_SNAPSHOT_MACOS_PREFS:-true}" == "true" ]]; then