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>

apps/learn/halkyon_learn_flutter/test/feature/auth/widget/password_set_up_test.dart

@@ -113,6 +113,12 @@ void main() {
     await openProfile(tester);
 
     expect(find.textContaining('Sign-in with GitHub ends on'), findsOneWidget);
+    // Without an address, the account goes at the end (the full reading's
+    // M1).
+    expect(
+      find.textContaining('will be deleted then, unless you set up a password'),
+      findsOneWidget,
+    );
     expect(find.text('Set up a password'), findsOneWidget);
   });