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/widget/account_tiles_test.dart

@@ -132,7 +132,7 @@ void main() {
   testWidgets('deleting an identified account while the instance is out of '
       'reach says so, and deletes nothing (the short reading)', (tester) async {
     gateway = FakeAuthGateway(
-      status: const SignedIn(authUserId: 'github-1', isIdentified: true),
+      status: const SignedIn(authUserId: 'person-2', isIdentified: true),
     );
     await start(tester);
     dependencies.passwordGateway.failing = const PasswordFailed(
@@ -151,7 +151,7 @@ void main() {
       findsOneWidget,
     );
     expect(gateway.accountDeletions, 0);
-    expect(find.text('App of github-1'), findsOneWidget);
+    expect(find.text('App of person-2'), findsOneWidget);
   });
 
   testWidgets('deleting an account whose setup never finished asks for the '
@@ -353,7 +353,7 @@ void main() {
   testWidgets("a sign-out the device's storage failed keeps the person in "
       'their app, and says so', (tester) async {
     gateway = FakeAuthGateway(
-      status: const SignedIn(authUserId: 'github-1', isIdentified: true),
+      status: const SignedIn(authUserId: 'person-2', isIdentified: true),
     )..signOutError = Exception('The keyring is locked');
     await start(tester);
 
@@ -364,7 +364,7 @@ void main() {
       find.text("Couldn't sign out on this device. Try again."),
       findsOneWidget,
     );
-    expect(find.text('App of github-1'), findsOneWidget);
+    expect(find.text('App of person-2'), findsOneWidget);
   });
 
   testWidgets('speaks German', (tester) async {