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_base_server/test/integration/test_tools/serverpod_test_tools.dart

@@ -204,8 +204,6 @@ class TestEndpoints {
 
   late final _EmailCodeEndpoint emailCode;
 
-  late final _GitHubSignInEndpoint gitHubSignIn;
-
   late final _InstanceEndpoint instance;
 
   late final _KeysEndpoint keys;
@@ -240,10 +238,6 @@ class _InternalTestEndpoints extends TestEndpoints
       endpoints,
       serializationManager,
     );
-    gitHubSignIn = _GitHubSignInEndpoint(
-      endpoints,
-      serializationManager,
-    );
     instance = _InstanceEndpoint(
       endpoints,
       serializationManager,
@@ -407,54 +401,6 @@ class _EmailCodeEndpoint {
   }
 }
 
-class _GitHubSignInEndpoint {
-  _GitHubSignInEndpoint(
-    this._endpointDispatch,
-    this._serializationManager,
-  );
-
-  final _is.EndpointDispatch _endpointDispatch;
-
-  final _is.SerializationManager _serializationManager;
-
-  _ida.Future<_iacs.AuthSuccess> signIn(
-    _ist.TestSessionBuilder sessionBuilder, {
-    required String code,
-    required String codeVerifier,
-    required String redirectUri,
-  }) async {
-    return _ist.callAwaitableFunctionAndHandleExceptions(() async {
-      var _localUniqueSession =
-          (sessionBuilder as _ist.InternalTestSessionBuilder).internalBuild(
-            endpoint: 'gitHubSignIn',
-            method: 'signIn',
-          );
-      try {
-        var _localCallContext = await _endpointDispatch.getMethodCallContext(
-          createSessionCallback: (_) => _localUniqueSession,
-          endpointPath: 'gitHubSignIn',
-          methodName: 'signIn',
-          parameters: _ist.testObjectToJson({
-            'code': code,
-            'codeVerifier': codeVerifier,
-            'redirectUri': redirectUri,
-          }),
-          serializationManager: _serializationManager,
-        );
-        var _localReturnValue =
-            await (_localCallContext.method.call(
-                  _localUniqueSession,
-                  _localCallContext.arguments,
-                )
-                as _ida.Future<_iacs.AuthSuccess>);
-        return _localReturnValue;
-      } finally {
-        await _localUniqueSession.close();
-      }
-    });
-  }
-}
-
 class _InstanceEndpoint {
   _InstanceEndpoint(
     this._endpointDispatch,