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/instance/instance_test.dart

@@ -1,4 +1,3 @@
-import 'package:clock/clock.dart';
 import 'package:halkyon_base_server/base.dart';
 import 'package:serverpod/serverpod.dart';
 import 'package:test/test.dart';
@@ -73,35 +72,10 @@ void main() {
       expect(basics.backupLocation, isNull);
     });
 
-    test('offers no GitHub sign-in without an OAuth app of its own', () {
-      // The tests' passwords hold no githubClientId.
-      expect(basics.gitHubClientId, isNull);
-    });
-
-    test('says when GitHub sign-in ends, until it has', () async {
+    test('serves no GitHub sign-in: GitHub signs nobody in any more (ADR '
+        '0020, D36)', () {
       final pod = sessionBuilder.build().serverpod;
-      final unauthenticated = sessionBuilder.copyWith(
-        authentication: AuthenticationOverride.unauthenticated(),
-      );
-      addTearDown(() => useGitHubSignInEnds(pod, null));
-      expect(basics.gitHubSignInEnds, isNull);
-
-      final ends = DateTime.utc(2027, 1, 22);
-      useGitHubSignInEnds(pod, ends);
-      expect(
-        (await withClock(
-          Clock.fixed(DateTime.utc(2026, 10, 22)),
-          () => endpoints.instance.basics(unauthenticated),
-        )).gitHubSignInEnds,
-        ends,
-      );
-      expect(
-        (await withClock(
-          Clock.fixed(ends),
-          () => endpoints.instance.basics(unauthenticated),
-        )).gitHubSignInEnds,
-        isNull,
-      );
+      expect(pod.server.endpoints.getConnectorByName('gitHubSignIn'), isNull);
     });
 
     test('offers email sign-in only where it can mail codes', () async {