feat(instance): the basics say when GitHub sign-in ends

ADR 0017, D2: GitHub sign-in is deprecated. Where the operator set
`GITHUB_SIGN_IN_ENDS`, `instance.basics()` names the day until it has
come, so apps can tell people who sign in with GitHub to add an
address, a password and an authenticator before then.

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/test/integration/instance/instance_test.dart

@@ -1,3 +1,4 @@
+import 'package:clock/clock.dart';
 import 'package:halkyon_base_server/base.dart';
 import 'package:serverpod/serverpod.dart';
 import 'package:test/test.dart';
@@ -68,6 +69,32 @@ void main() {
       expect(basics.gitHubClientId, isNull);
     });
 
+    test('says when GitHub sign-in ends, until it has', () async {
+      final pod = sessionBuilder.build().serverpod;
+      final unauthenticated = sessionBuilder.copyWith(
+        authentication: AuthenticationOverride.unauthenticated(),
+      );
+      addTearDown(() => useGitHubSignInEnds(pod, null));
+      expect(basics.gitHubSignInEnds, isNull);
+
+      final ends = DateTime.utc(2027, 1, 22);
+      useGitHubSignInEnds(pod, ends);
+      expect(
+        (await withClock(
+          Clock.fixed(DateTime.utc(2026, 10, 22)),
+          () => endpoints.instance.basics(unauthenticated),
+        )).gitHubSignInEnds,
+        ends,
+      );
+      expect(
+        (await withClock(
+          Clock.fixed(ends),
+          () => endpoints.instance.basics(unauthenticated),
+        )).gitHubSignInEnds,
+        isNull,
+      );
+    });
+
     test('offers email sign-in only where it can mail codes', () async {
       final codes = EmailCodes.of(sessionBuilder.build().serverpod);
       final unauthenticated = sessionBuilder.copyWith(