macos: set the update URL

macos/Sources/Features/Update/UpdateDelegate.swift

@@ -2,7 +2,10 @@ import Sparkle
 
 class UpdaterDelegate: NSObject, SPUUpdaterDelegate {
     func feedURLString(for updater: SPUUpdater) -> String? {
-        // TODO
-        return nil
+        // Eventually w want to support multiple channels. Sparkle itself supports
+        // channels but we probably don't want some appcasts in the same file (i.e.
+        // tip) so this would be the place to change that. For now, we hardcode the
+        // tip appcast URL since it is all we support.
+        return "https://tip.files.ghostty.dev/appcast.xml"
     }
 }