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_server/lib/src/generated/future_calls.dart

@@ -222,10 +222,8 @@ class _PruningFutureCallDispatcher {
   }
 }
 
-/// Prunes one batch of anonymous accounts and logs how many people went;
-/// 30 days after GitHub sign-in ended (`GITHUB_SIGN_IN_ENDS`), deletes the
-/// accounts that had no other way in and forgets every GitHub ID too, on
-/// the register; and forgets the resets that lapsed (ADR 0017).
+/// Prunes one batch of anonymous accounts and logs how many people went,
+/// and forgets the resets that lapsed (ADR 0017).
 class PruningPruneAccountsFutureCall extends _is.FutureCall
     implements _is.InvokableFutureCall {
   @override

apps/todo/halkyon_todo_server/lib/src/pruning_future_call.dart

@@ -10,18 +10,14 @@ import 'package:serverpod/serverpod.dart';
 /// again of the accounts gone, hourly; as a member, deleting the people
 /// whose account the register no longer knows, daily.
 class PruningFutureCall extends FutureCall {
-  /// Prunes one batch of anonymous accounts and logs how many people went;
-  /// 30 days after GitHub sign-in ended (`GITHUB_SIGN_IN_ENDS`), deletes the
-  /// accounts that had no other way in and forgets every GitHub ID too, on
-  /// the register; and forgets the resets that lapsed (ADR 0017).
+  /// Prunes one batch of anonymous accounts and logs how many people went,
+  /// and forgets the resets that lapsed (ADR 0017).
   Future<void> pruneAccounts(Session session) async {
     final now = clock.now();
     final pruned = await pruneAnonymousPeople(session);
-    final gitHub = await forgetGitHubAccounts(session, now: now);
     final resets = await forgetLapsedResets(session, now: now);
     session.log(
-      'Pruned $pruned anonymous people, $gitHub GitHub IDs and '
-      '$resets lapsed resets.',
+      'Pruned $pruned anonymous people and $resets lapsed resets.',
     );
   }