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/lib/l10n/halkyon_app_localizations_en.dart

@@ -173,31 +173,6 @@ class HalkyonAppLocalizationsEn extends HalkyonAppLocalizations {
     return 'Start on $instance:';
   }
 
-  @override
-  String get startWithGitHub => 'Sign in with GitHub';
-
-  @override
-  String get startGitHubFailed => 'Couldn\'t sign in with GitHub.';
-
-  @override
-  String get startNoGitHubAccount =>
-      'No account here signs in with this GitHub account.';
-
-  @override
-  String get gitHubCreateAccountInstead =>
-      'Create an account with your email address and a password instead.';
-
-  @override
-  String get gitHubSimpleLoginInstead =>
-      'Continue with SimpleLogin to create one instead.';
-
-  @override
-  String get gitHubUsePassword =>
-      'This account signs in with its email address and password now, not with GitHub.';
-
-  @override
-  String get gitHubEnded => 'Sign-in with GitHub has ended on this instance.';
-
   @override
   String get startOtherInstance => 'Use another instance';
 
@@ -274,11 +249,6 @@ class HalkyonAppLocalizationsEn extends HalkyonAppLocalizations {
   @override
   String get startCreateAccount => 'Create an account';
 
-  @override
-  String startGitHubEnds(String date) {
-    return 'Sign-in with GitHub ends on $date.';
-  }
-
   @override
   String get passwordAddress => 'Email address';
 
@@ -584,11 +554,6 @@ class HalkyonAppLocalizationsEn extends HalkyonAppLocalizations {
   String get passwordSectionFailed =>
       'Couldn\'t read how your account signs in.';
 
-  @override
-  String passwordSectionGitHubEnds(String date) {
-    return 'Sign-in with GitHub ends on $date. Set up a password before then to keep your account.';
-  }
-
   @override
   String get passwordSectionSetUp => 'Set up a password';
 
@@ -697,9 +662,6 @@ class HalkyonAppLocalizationsEn extends HalkyonAppLocalizations {
   @override
   String get passwordAddressTaken => 'This address belongs to another account.';
 
-  @override
-  String get passwordNoAddress => 'Your account has no email address yet.';
-
   @override
   String get passwordNoPassword => 'This account has no password.';
 
@@ -816,23 +778,6 @@ class HalkyonAppLocalizationsEn extends HalkyonAppLocalizations {
   @override
   String get passwordDeleteSendCode => 'Email me a code';
 
-  @override
-  String get passwordSignInAgain =>
-      'To give an account that signs in with GitHub its password, sign in with GitHub again first: it has been more than a few minutes.';
-
-  @override
-  String get passwordSignInWithGitHubAgain => 'Sign in with GitHub again';
-
-  @override
-  String get passwordOfferTitle => 'Set up a password';
-
-  @override
-  String get passwordOfferBody =>
-      'Sign-in with GitHub is ending. Set up a password and an authenticator app to keep your account.';
-
-  @override
-  String get passwordOfferLater => 'Later';
-
   @override
   String get passwordAuthenticatorPasswordIntro =>
       'To add it, confirm it is you with your password.';
@@ -874,11 +819,6 @@ class HalkyonAppLocalizationsEn extends HalkyonAppLocalizations {
   String get passwordCallOffIntro =>
       'Your password and a code call off every reset waiting.';
 
-  @override
-  String passwordSectionGitHubEndsDeletes(String date) {
-    return 'Sign-in with GitHub ends on $date. Set up a password before then: an account without one is deleted 30 days later, with everything in it.';
-  }
-
   @override
   String get passwordSignedInBefore =>
       'Only a device signed in since then can dismiss this warning: sign in again on this one, or change your password.';