chore(release): tool/release.sh goes, and the snap goes to the Store's edge

Build 21 (v0.18.0) was the release tool/release.sh was kept for, the one
going with ADR 0014's breaking deploy; every release since is
`dart run tool/release.dart learn`. The comments that named the script
(the F-Droid script, the certificate pin, the Gradle signing) now name
the Dart tool.

Learn's release uploads the snap to the Snap Store's edge channel last,
where v0.18.0 went by hand; the release refuses to start unless
`snapcraft whoami` shows a login.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/android/app/build.gradle.kts

@@ -28,7 +28,7 @@ android {
         versionName = flutter.versionName
     }
 
-    // The app's own key signs release builds when tool/release.sh hands it over
+    // The app's own key signs release builds when tool/release.dart hands it over
     // through the environment: the keystore's path and the key's alias, and the
     // password asked for at each release, never written down. Other release builds
     // are signed with the debug key, so `flutter run --release` works anywhere; the