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>

apps/todo/halkyon_todo_flutter/test/app/todo_app_test.dart

@@ -133,25 +133,14 @@ void main() {
     );
   });
 
-  testWidgets('signed out, it asks how to start; a GitHub account nobody here '
-      'signs in with is said so, with an account to create in its place '
-      'where passwords are offered (ADR 0017)', (tester) async {
+  testWidgets('signed out, it asks how to start, with an account to create '
+      'where passwords are offered (ADR 0017), and no GitHub (ADR 0020, '
+      'D36)', (tester) async {
     await start(tester, usedBefore: true, signedOut: true);
 
-    expect(find.text('Sign in with GitHub'), findsOneWidget);
-    gateway.gitHubError = const GitHubSignInFailed(
-      GitHubSignInProblem.unknownAccount,
-    );
-    await tester.tap(find.text('Sign in with GitHub'));
-    await tester.pumpAndSettle();
-
-    expect(
-      find.text(
-        'No account here signs in with this GitHub account. Create an '
-        'account with your email address and a password instead.',
-      ),
-      findsOneWidget,
-    );
+    expect(find.text('Start on todo.example.org:'), findsOneWidget);
+    expect(find.text('Create an account'), findsOneWidget);
+    expect(find.textContaining('GitHub'), findsNothing);
   });
 
   testWidgets('where no account can be made, it says so', (tester) async {
@@ -161,16 +150,6 @@ void main() {
       find.text('todo.example.org offers no way to create an account.'),
       findsOneWidget,
     );
-    gateway.gitHubError = const GitHubSignInFailed(
-      GitHubSignInProblem.unknownAccount,
-    );
-    await tester.tap(find.text('Sign in with GitHub'));
-    await tester.pumpAndSettle();
-
-    expect(
-      find.text('No account here signs in with this GitHub account.'),
-      findsOneWidget,
-    );
   });
 
   testWidgets('the account says on which instance it is signed in; signing '
@@ -187,7 +166,7 @@ void main() {
     expect(gateway.signOuts, 1);
     expect(gateway.accountDeletions, 0);
     expect(gateway.status, const SignedOut());
-    expect(find.text('Sign in with GitHub'), findsOneWidget);
+    expect(find.text('Start on todo.example.org:'), findsOneWidget);
   });
 
   testWidgets('where the instance offers SimpleLogin, the account sheet '
@@ -220,7 +199,7 @@ void main() {
     await tester.pumpAndSettle();
 
     expect(gateway.accountDeletions, 1);
-    expect(find.text('Sign in with GitHub'), findsOneWidget);
+    expect(find.text('Start on todo.example.org:'), findsOneWidget);
   });
 
   testWidgets('a deletion the instance never got says so on the account '