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/auth/data/serverpod_auth_gateway_test.dart

@@ -19,13 +19,9 @@ final class _MemoryStorage() implements ClientAuthSuccessStorage {
   Future<void> set(AuthSuccess? data) async => _session = data;
 }
 
-/// An instance's halkyon_base as the network shows it: its basics offer
-/// GitHub while it has [gitHubClientId], and a sign-up makes Ben, signed
-/// in with `ben-token`.
+/// An instance's halkyon_base as the network shows it: a sign-up makes
+/// Ben, signed in with `ben-token`.
 final class _Instance() {
-  /// The client ID of its GitHub OAuth app; none without.
-  String? gitHubClientId = 'github-client';
-
   static final anonymous = UuidValue.withValidation(
     '0191f0c4-1111-7000-8000-000000000001',
   );
@@ -41,20 +37,6 @@ final class _Instance() {
     calls.add((call: call, session: request.headers['authorization']));
     String sent(Object? body) => jsonEncode(body);
     return switch (call) {
-      'halkyon_base.instance/basics' => http.Response(
-        sent(
-          InstanceBasics(
-            app: 'learn',
-            gitHubClientId: gitHubClientId,
-            emailSignIn: true,
-            sourceCodeUrl: 'https://code.example.org/learn',
-            minimumAppBuild: 1,
-            passwordSignIn: true,
-            simpleLogin: true,
-          ).toJson(),
-        ),
-        200,
-      ),
       'halkyon_base.passwordSignIn/signUp' => http.Response(
         sent(_session(ben, 'ben-token', identified: true)),
         200,
@@ -91,13 +73,7 @@ void main() {
             storage: _MemoryStorage(),
           );
     await app.auth.initialize();
-    gateway = ServerpodAuthGateway(
-      app,
-      base: app.modules,
-      callbackScheme: 'app.halkyon.test',
-      appName: 'Halkyon Test',
-      httpClient: instance.client,
-    );
+    gateway = ServerpodAuthGateway(app, base: app.modules);
     addTearDown(() async {
       await gateway.dispose();
       app.close();
@@ -134,33 +110,6 @@ void main() {
     );
   });
 
-  test('tells from the basics whether the instance signs in with GitHub '
-      'those who have it', () async {
-    expect(await gateway.gitHubSignInOffered(), isTrue);
-
-    instance.gitHubClientId = null;
-    expect(await gateway.gitHubSignInOffered(), isFalse);
-  });
-
-  test("puts the instance's refusals of GitHub as the app's problems", () {
-    expect(
-      {
-        for (final reason in GitHubSignInRefusal.values)
-          reason: gitHubSignInProblemOf(reason),
-      },
-      {
-        GitHubSignInRefusal.unavailable: GitHubSignInProblem.refused,
-        GitHubSignInRefusal.takenByAnotherPerson: GitHubSignInProblem.refused,
-        GitHubSignInRefusal.otherGitHubAccountLinked:
-            GitHubSignInProblem.refused,
-        GitHubSignInRefusal.unknownAccount: GitHubSignInProblem.unknownAccount,
-        GitHubSignInRefusal.usePassword: GitHubSignInProblem.usePassword,
-        GitHubSignInRefusal.deprecated: GitHubSignInProblem.deprecated,
-        GitHubSignInRefusal.ended: GitHubSignInProblem.ended,
-      },
-    );
-  });
-
   test('a session granted elsewhere (a sign-up with a password, signed '
       'out) takes its place', () async {
     final passwords = ServerpodPasswordGateway(