ci: add GitHub Actions workflow to build Omega Mac app

Adds a manually-triggered workflow that builds the SwiftUI app on a
macOS 15 runner with Xcode 16, packages it as a .app bundle, and
uploads it as a downloadable artifact. Also lowers the deployment
target to macOS 14 so it runs on Sonoma.

apps/omega-mac/Package.swift

@@ -3,7 +3,7 @@ import PackageDescription
 
 let package = Package(
     name: "OmegaMac",
-    platforms: [.macOS(.v15)],
+    platforms: [.macOS(.v14)],
     dependencies: [
         .package(url: "https://github.com/gonzalezreal/swift-markdown-ui", from: "2.4.0"),
     ],