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/simplelogin/bloc/simplelogin_cubit_test.dart

@@ -516,9 +516,9 @@ void main() {
   group("SimpleLogin added to the account of the device's person", () {
     test('a person without keys keeps those their device holds, and their '
         'session, once the keys are set', () async {
-      await auth.switchToGitHub();
+      auth.signIn('ada');
       final deviceKeys = crypto.keysNumbered(1);
-      keys.kept['github-person'] = deviceKeys;
+      keys.kept['ada'] = deviceKeys;
       final cubit = cubitFor(link: true);
 
       await cubit.started();
@@ -529,9 +529,9 @@ void main() {
       expect(cubit.state, at(SimpleLoginStep.done));
       expect(
         auth.status,
-        const SignedIn(authUserId: 'github-person', isIdentified: true),
+        const SignedIn(authUserId: 'ada', isIdentified: true),
       );
-      expect(keys.kept, {'github-person': deviceKeys});
+      expect(keys.kept, {'ada': deviceKeys});
       expect(auth.usedSessions, isEmpty);
     });