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>

config/extra-installers.txt.example

@@ -0,0 +1,22 @@
+# extra-installers.txt — additional installers not available via Homebrew or MAS.
+#
+# Copy this file to ~/.config/ssr/extra-installers.txt and add your URLs.
+# It is backed up to the cloud alongside your Brewfile on every `ssr sync`.
+#
+# Format (one entry per line):
+#   [optional-name ]<url>
+#
+# Supported types: .dmg  .pkg  .zip
+# Lines starting with # and blank lines are ignored.
+#
+# The installer is skipped if an app with a matching name is already
+# present in /Applications or ~/Applications.
+#
+# ─────────────────────────────────────────────────────────────────────────────
+
+# EasyMCP Desktop (Adobe AI Foundations)
+EasyMCP https://github.com/Adobe-AIFoundations/easymcp/releases/download/v3.0.3/EasyMCP-Desktop-3.0.3-arm64.dmg
+
+# Add your own entries below:
+# SomeApp https://example.com/SomeApp-1.2.dmg
+# AnotherTool https://builds.example.com/tool-latest.pkg