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>

AGENTS.md

@@ -9,7 +9,7 @@ Halkyon's apps and the packages they share, in one repository and one pub worksp
 - **`apps/<app>/`**: each app's packages, as `serverpod create` lays them out (`halkyon_<app>_server`, `_client`, `_flutter`), plus what is the app's alone (Learn's curator, the scripts writing Learn's assets in `apps/learn/tool/`, run from `apps/learn`). Learn is the app that runs; Todo is still Serverpod's scaffold; Train and Meditate are to come.
 - **`packages/`**: what the apps share, each with its README and licence, which the apps take by path (`path: ../../../packages/<package>`), so a change to a shared package and to the apps using it is one commit:
   - `halkyon_ui`: Halkyon's look, the theme around cobalt and copper, Noto Sans bundled, the window-size classes and the widgets every screen is built of, with texts of its own (English, German).
-  - `halkyon_base_server`, `halkyon_base_client`: the Serverpod module every Halkyon server includes (ADR 0014): signing in (a password with TOTP and the resets of ADR 0017, SimpleLogin on `halkyon_oidc`, email codes proving an address, GitHub until it ends; no anonymous accounts since ADR 0020, D25), an account's ways in and its deletion with a hook for each app's own data, the pruning, the instance's basics, people's handles and public profiles, and the instance's register (ADR 0016): one account per person across an operator's services, held by one service (Learn's server on learn.halkyon.eu) and asked by the others, its members (`ACCOUNT_REGISTER_URL`), all sharing a secret (`INSTANCE_SECRET`); its README says how.
+  - `halkyon_base_server`, `halkyon_base_client`: the Serverpod module every Halkyon server includes (ADR 0014): signing in (a password with TOTP and the resets of ADR 0017, SimpleLogin on `halkyon_oidc`, email codes proving an address; no anonymous accounts since ADR 0020, D25, nor GitHub since D36), an account's ways in and its deletion with a hook for each app's own data, the pruning, the instance's basics, people's handles and public profiles, and the instance's register (ADR 0016): one account per person across an operator's services, held by one service (Learn's server on learn.halkyon.eu) and asked by the others, its members (`ACCOUNT_REGISTER_URL`), all sharing a secret (`INSTANCE_SECRET`); its README says how.
   - `halkyon_app`: every app's shell on the module's client (ADR 0014): the instances a device keeps accounts on and the first start, signing in and switching person, the gates building an app's dependencies per instance and per person, the sync service and where it stands, the account section of an app's settings (`AccountTiles`), About with the instance's source link (`showAppAbout`), the language every app picks (`resolveAppLocale`) and countries' and subdivisions' names (Unicode CLDR's, bundled), with texts that name the app.
   - `halkyon_core` (low-level primitives in pure Dart: ids, a logger, `SerialQueue`, `CoalescingRunner`) and `halkyon_interop` (contracts between apps, pure Dart).
   - `halkyon_keys` (pure Dart on libsodium, ADR 0017): an account's keys on the device, the password's Argon2id and SRP's x, the key pairs and their wrapping, the recovery phrase, data keys, content sealed and signed. SRP-6a's two halves are the module's (`halkyon_base_server`'s `base.dart`, `halkyon_base_client`'s `srp.dart`), as are TOTP and the recovery codes. `sodium`'s build hook compiles libsodium, so every machine that builds or tests needs a C compiler and `make`.