Add Sparkle auto-update support to a native macOS app. Use this skill whenever the user wants to add auto-updates, implement Sparkle, set up an appcast feed, add "Check for Updates" to their app, or integrate automatic update checking. Also trigger when the user mentions Sparkle, appcast, SUFeedURL, EdDSA signing, or update notifications. This covers the full setup: adding the Sparkle SPM dependency, creating the UpdaterManager singleton, wiring it into the app delegate and UI, configuring Info.plist, and generating the initial appcast.xml.