fix(auth): GitHub-only accounts go at GitHub's end; only the anonymous add an address

The full reading's M1 and L6: once GitHub IDs were forgotten at
GITHUB_SIGN_IN_ENDS, an account that had signed in with GitHub alone
counted as anonymous, so a stolen session and any mailbox gave it a first
password and authenticator; and such accounts, with no way in left, were
never pruned. Now:

- at the end, the daily run that forgets the GitHub IDs first deletes the
  accounts that sign in with GitHub alone (no address, so never a
  password), with everything they have, members told (deletePerson); the
  app tells them for the three months before that their account goes then
  unless they set up a password (passwordSectionGitHubEndsDeletes, en/de);
- password.set adds an address without GitHub only for someone anonymous
  where the session lives (anonymousHere, which a member tells the
  register, as it tells freshGitHubSignIn); anyone else is refused before
  any code is used.

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_base_server/lib/src/register/instance_api.dart

@@ -233,8 +233,11 @@ Future<Map<String, Object?>?> _answer(
         setup: arguments.model('setup', PasswordSetup.fromJson),
         request: arguments.optionalId('request'),
         code: arguments.optionalString('code'),
-        // The member tells it of its own session (the fourth reading).
+        // The member tells it of its own session (the fourth reading) and
+        // person (the full reading's M1): the register keeps no anonymous
+        // person of a member's.
         freshGitHubSignIn: arguments.flag('freshGitHubSignIn'),
+        anonymousHere: arguments.flag('anonymousHere'),
       );
       return const {};
     case RegisterOperation.beginPasswordSignIn: