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>

deploy/.env.example

@@ -14,9 +14,10 @@ DOMAIN=http://localhost
 #ACCOUNT_DOMAIN=halkyon.eu
 
 # The instance's other services (optional, ADR 0016), such as Halkyon Todo: Halkyon
-# Learn holds the instance's register of accounts, so an address, a GitHub account and a
-# handle are one person in every service, and deleting the account deletes it in all.
-# The register learns when someone signs in to which service (the privacy page says so).
+# Learn holds the instance's register of accounts, so an address, a SimpleLogin account
+# and a handle are one person in every service, and deleting the account deletes it in
+# all. The register learns when someone signs in to which service (the privacy page says
+# so).
 # INSTANCE_SECRET is the secret the services share, carried by every call between them:
 # the same value in each service's environment, never given to a device. Empty: no
 # service calls another. Generate: openssl rand -hex 32
@@ -49,13 +50,6 @@ POSTGRES_PASSWORD=
 # Generate: openssl rand -base64 32
 SESSION_KEY_HASH_PEPPER=
 
-# GitHub sign-in (optional): the client ID and secret of this instance's own GitHub
-# OAuth app (GitHub → Settings → Developer settings → OAuth Apps), registered with the
-# callback URLs app.halkyon.learn://github-callback and http://127.0.0.1/github-callback.
-# Empty: GitHub sign-in is off, and the app doesn't offer it.
-GITHUB_CLIENT_ID=
-GITHUB_CLIENT_SECRET=
-
 # Email sign-in (optional, ADR 0010): people sign in with a code mailed to their address,
 # of which the instance keeps only a keyed hash. Off unless the pepper and all of the SMTP
 # service's settings are set; the app then doesn't offer it.
@@ -108,11 +102,6 @@ SIMPLELOGIN_CLIENT_SECRET=
 # server as it starts.
 #SIGN_IN_WAYS=simplelogin
 
-# The day GitHub stops signing anyone in, three months after the release that brings
-# passwords (ADR 0017, D2): from then on its people sign in with their address and
-# password only, and the daily pruning forgets every GitHub ID. Empty: GitHub goes on.
-#GITHUB_SIGN_IN_ENDS=2027-01-22
-
 # Where the source of what this instance runs is, shown in the app's About dialog: both
 # licences (AGPL for the app, SSPL for the server) require offering it to the people who
 # use the instance. Empty: the project's repository, which is right for unmodified code.