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/pubspec.yaml

@@ -29,10 +29,6 @@ dependencies:
   # The account's private keys, once unlocked, in the system's keyring
   # (ADR 0017), as Serverpod keeps sessions.
   flutter_secure_storage: ^10.0.0
-  # GitHub's sign-in goes through the package's front, so LoopbackWebAuth
-  # (halkyon_oidc_flutter) takes the place of its browser half on Linux and
-  # Windows.
-  flutter_web_auth_2_platform_interface: ^5.0.0
   halkyon_base_client:
     path: ../halkyon_base_client
   halkyon_core:
@@ -74,8 +70,6 @@ dev_dependencies:
   fake_async: ^1.3.3
   flutter_test:
     sdk: flutter
-  # A test drives sign-in through its front, as Serverpod's GitHub sign-in does.
-  flutter_web_auth_2: ^5.1.0
   # The register's own checks, which the device's real cryptography is held
   # to (register_interop_test.dart).
   halkyon_base_server: