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/base/base_client_test.dart

@@ -12,8 +12,6 @@ import 'package:http/testing.dart';
 String sent(SerializableException refusal) => jsonEncode({
   'className': switch (refusal) {
     EmailSignInRefusedException() => 'halkyon_base.EmailSignInRefusedException',
-    GitHubSignInRefusedException() =>
-      'halkyon_base.GitHubSignInRefusedException',
     PasswordRefusedException() => 'halkyon_base.PasswordRefusedException',
     OidcRefusedException() => 'halkyon_oidc.OidcRefusedException',
     _ => throw ArgumentError.value(refusal),
@@ -41,15 +39,13 @@ void main() {
     expect(
       protocol.decodeWithType(
         sent(
-          GitHubSignInRefusedException(
-            reason: GitHubSignInRefusal.unknownAccount,
-          ),
+          PasswordRefusedException(reason: PasswordRefusal.wrongCredentials),
         ),
       ),
-      isA<GitHubSignInRefusedException>().having(
+      isA<PasswordRefusedException>().having(
         (refusal) => refusal.reason,
         'reason',
-        GitHubSignInRefusal.unknownAccount,
+        PasswordRefusal.wrongCredentials,
       ),
     );
   });