feat(base,app)!: GitHub signs nobody in any more (ADR 0020, D36)

Every account signs in with SimpleLogin or a password now, so GitHub
goes from the module, the shared app package, Learn and Todo, ahead of
ADR 0017's GITHUB_SIGN_IN_ENDS.

- halkyon_base: no `gitHubSignIn` endpoint, GitHub identity provider or
  GitHub refusals; no `githubClientId`/`githubClientSecret`, no
  `GITHUB_SIGN_IN_ENDS` and no `forgetGitHubAccounts`; the register
  loses `account-with-github`. `SignInWays.gitHub`, the basics'
  `gitHubClientId` and `gitHubSignInEnds`, and `PasswordRefusal`'s
  `signInAgain` and `noAddress` go. A first password takes a code
  mailed to the person's own address; an account without one sets none
  (`wrongCredentials`).
- Learn's server: `instance.info()` without `gitHubClientId`, the
  pruning without GitHub, and `minimumAppBuild` 24, as builds up to 23
  (v0.20.0) require `SignInWays.gitHub`: the server deploys with the
  release of build 24.
- halkyon_app: no GitHub button or problems on the start screen, no
  password offered after a GitHub sign-in, and `ServerpodAuthGateway`
  without the browser half only GitHub used (`(client, base:)`). The
  account section offers a first password only to an account with an
  address. `LoopbackWebAuth` and the apps' schemes stay, SimpleLogin's
  ways back.
- Learn's Profile lists no GitHub; the texts go (English, German).
- Serverpod's `serverpod_auth_idp_github_account` stays, as its
  migrations make it; a row goes with its account, and ADR 0020's D36
  gives operators the SQL to delete those left.
- deploy/: compose.yaml and .env.example without GitHub. The READMEs,
  AGENTS.md files and ADR 0017 amended.

BREAKING CHANGE: builds up to 23 can't read the instance's answers
about an account's ways in; minimumAppBuild is 24.

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

packages/halkyon_app/test/password/widget/password_sheets_test.dart

@@ -575,17 +575,36 @@ void main() {
       await tester.pumpAndSettle();
     }
 
-    testWidgets('offers one signed in with GitHub a password (ADR 0017, '
-        'D2)', (tester) async {
-      await gateway.switchToGitHub();
+    /// Signs in the person of an account at gina@example.org without a
+    /// password, as codes by mail left accounts; their id.
+    String signInGina() {
+      final person = dependencies.passwordGateway
+          .addAddressOnly('gina@example.org')
+          .person;
+      gateway.signIn(person);
+      return person;
+    }
+
+    testWidgets('offers a password to one signed in with an address and '
+        'none yet', (tester) async {
+      signInGina();
       await show(tester);
 
       expect(find.text('Set up a password'), findsOneWidget);
       expect(find.text('Change password'), findsNothing);
     });
 
-    /// Sets up a password for the GitHub account signed in, up to the step
-    /// after the phrase.
+    testWidgets('offers none to one without an address: a code mailed to '
+        'their own is the only proof (ADR 0020, D36)', (tester) async {
+      gateway.signIn('ada');
+      await show(tester);
+
+      expect(find.text('Set up a password'), findsNothing);
+      expect(find.text('Change password'), findsNothing);
+    });
+
+    /// Sets up a password for the person signed in, up to the step after
+    /// the phrase, typing the address the device doesn't know.
     Future<void> setUpPassword(WidgetTester tester) async {
       await show(tester);
       await tester.tap(find.text('Set up a password'));
@@ -600,10 +619,9 @@ void main() {
       await tapButton(tester, 'Continue');
     }
 
-    testWidgets('asks one signed in with GitHub without a handle for their '
-        'first with the password (ADR 0020, D28)', (tester) async {
-      await gateway.switchToGitHub();
-      final person = (gateway.status as SignedIn).authUserId;
+    testWidgets('asks one signed in without a handle for their first with '
+        'the password (ADR 0020, D28)', (tester) async {
+      final person = signInGina();
       await setUpPassword(tester);
 
       await tester.enterText(field('Handle'), 'gina');
@@ -622,8 +640,7 @@ void main() {
     });
 
     testWidgets('asks no handle of one who has one', (tester) async {
-      await gateway.switchToGitHub();
-      final person = (gateway.status as SignedIn).authUserId;
+      final person = signInGina();
       dependencies.passwordGateway.handles[person] = 'gina';
       await setUpPassword(tester);