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/password/data/password_repository_test.dart

@@ -61,7 +61,7 @@ void main() {
     );
     addTearDown(locked.dispose);
 
-    await auth.switchToGitHub();
+    auth.signIn('ada');
     await settle();
     await auth.signOut();
     await settle();
@@ -367,13 +367,15 @@ void main() {
   });
 
   group('signed in', () {
-    test('one who signs in with GitHub given a password keeps the account, '
-        'the keys and the address kept (ADR 0017, D2)', () async {
-      await auth.switchToGitHub();
+    test('one with an address and no password given one keeps the account, '
+        'a code mailed there proving them, the keys and the address '
+        'kept', () async {
+      final dana = gateway.addAddressOnly('dana@example.org').person;
+      auth.signIn(dana);
       final request = await passwords.sendCode(
         'dana@example.org',
         language: 'en',
-        purpose: MailCodePurpose.signUp,
+        purpose: MailCodePurpose.firstPassword,
       );
 
       final material = await passwords.makePassword('her password');
@@ -396,13 +398,10 @@ void main() {
         handle: 'dana',
       );
 
-      expect(
-        auth.status,
-        const SignedIn(authUserId: 'github-person', isIdentified: true),
-      );
-      expect(keys.kept['github-person'], material.keys);
-      expect(addresses.addresses['github-person'], 'dana@example.org');
-      expect(gateway.handles['github-person'], 'dana');
+      expect(auth.status, SignedIn(authUserId: dana, isIdentified: true));
+      expect(keys.kept[dana], material.keys);
+      expect(addresses.addresses[dana], 'dana@example.org');
+      expect(gateway.handles[dana], 'dana');
     });
 
     test('someone signed up anew gets new keys, never the keys kept on the '
@@ -416,8 +415,8 @@ void main() {
 
     test("keeps only the signed-in person's keys: another signing in, or "
         'signing out, takes the others away', () async {
-      await auth.switchToGitHub();
-      await keys.keep('github-person', crypto.keysNumbered(1));
+      auth.signIn('ada');
+      await keys.keep('ada', crypto.keysNumbered(1));
       await keys.keep('someone-else', crypto.keysNumbered(2));
 
       await auth.signOut();