Integrate Sparkle auto-updates for macOS Developer ID builds

New Release-DeveloperID configuration gates Sparkle behind a SPARKLE
compile flag so the App Store Release build stays Sparkle-free. Adds
SPUStandardUpdaterController wrapper, Check for Updates menu command,
Advanced Settings section with beta channel toggle, and a Ruby script
plus GitHub Actions job that signs each release and publishes the
appcast to gh-pages for consumption by Sparkle and Homebrew cask.

Yattee/Views/macOS/CheckForUpdatesMenuItem.swift

@@ -0,0 +1,27 @@
+//
+//  CheckForUpdatesMenuItem.swift
+//  Yattee
+//
+//  "Check for Updates…" menu item shown in the macOS app menu for
+//  Developer ID builds. Wired to the Sparkle-backed `AppUpdater`.
+//
+//  Compiled in only when the `SPARKLE` Swift flag is set (i.e. the
+//  `Release-DeveloperID` configuration). See AGENTS.md.
+//
+
+#if SPARKLE && os(macOS)
+
+import SwiftUI
+
+struct CheckForUpdatesMenuItem: View {
+    @State private var updater = AppUpdater.shared
+
+    var body: some View {
+        Button(String(localized: "menu.app.checkForUpdates")) {
+            updater.checkForUpdates()
+        }
+        .disabled(!updater.canCheckForUpdates)
+    }
+}
+
+#endif