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/README.md

@@ -39,8 +39,7 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
   the instance in use (`AppDependencies`, handed down by `AppScope`), anew on
   a switch, the previous ones closed first; `SessionGate` shows the app of
   whoever is signed in and asks how to start while nobody is
-  (`StartScreen`: SimpleLogin, a password, GitHub for those who have it,
-  another instance; an instance out of reach is said so, with Try again,
+  (`StartScreen`: SimpleLogin, a password, another instance; an instance out of reach is said so, with Try again,
   and one offering no way to create an account too), at a first start as
   after signing out; an anonymous account's session, left from before
   there were none, is deleted as the app starts (ADR 0020, D25), falling
@@ -62,15 +61,13 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
   it `hasSnackBars`). An instance out of reach signs out on the device all
   the same (Serverpod's `signOutDevice`), so only the device's own storage
   fails a sign-out.
-- **Signing in** (`lib/src/auth/`): `AuthRepository` on an `AuthGateway`;
-  `ServerpodAuthGateway` signs in with GitHub those who have it, as it is
-  added to nobody any more (ADR 0020, D25; the system browser and a loopback
-  address on Linux and Windows, halkyon_oidc_flutter's `LoopbackWebAuth`),
-  through the instance's `halkyon_base`, and puts a session granted elsewhere
-  in place (`useSession`); `SessionBloc` and `SignInWaysCubit`;
-  `SignInAddresses`, `PersonFolders`. The start screen says why a GitHub
-  sign-in was refused, and for a GitHub account nobody here has, the way to
-  create an account in its place.
+- **The session** (`lib/src/auth/`): `AuthRepository` on an `AuthGateway`;
+  `ServerpodAuthGateway` follows the session the app's Serverpod client
+  keeps, signs out, deletes the account through the instance's
+  `halkyon_base`, and puts a session granted elsewhere in place
+  (`useSession`: a password's or SimpleLogin's sign-in; GitHub signs nobody
+  in since ADR 0020, D36); `SessionBloc` and `SignInWaysCubit`;
+  `SignInAddresses`, `PersonFolders`.
 - **Content sealed with data keys** (`lib/src/privacy/`, ADR 0019):
   `ContentCipher` (`SodiumContentCipher`, on the isolate it is used on, as an
   app seals and opens rows one by one; `FakeContentCipher` in tests) makes a
@@ -199,10 +196,8 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
    `GlobalMaterialLocalizations.delegates`.
 4. **The frame** (`AppFrame`): the app's name and wordmark word, its icon, an
    example address, its theme, delegates, languages (`resolveAppLocale` as
-   its resolution) and launch screen, and the three sentences only it can
-   say: what the first start's instance keeps of the person's, what a switch
-   to another account leaves behind, and where GitHub is added later (shown
-   only where the instance offers no passwords).
+   its resolution) and launch screen, and the sentence only it can say: what
+   the first start's instance keeps of the person's.
 5. **Android**: the app's `MainActivity` answers `app.halkyon/screen_guard`
    (copy Learn's or Todo's `ScreenGuard.kt` and call `ScreenGuard.attach` in
    `configureFlutterEngine`), or secrets show in screenshots and recents.
@@ -214,10 +209,7 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
    `SodiumAccountCrypto()`, `SecureAccountKeyStore(storage.keysOf(instance))`,
    the auth gateway and the app's `SignInAddresses`. Its `auth` is an
    `AuthRepository` on `ServerpodAuthGateway(client, base:
-   client.modules.base, callbackScheme: …, appName: …)`; the scheme is the
-   app's own (`app.halkyon.learn`), registered with each platform and in the
-   instance's GitHub OAuth app, whose loopback address on Linux and Windows is
-   `http://127.0.0.1/github-callback` (port 43823 unless given).
+   client.modules.base)`.
 7. **The tree**:
 
    ```dart