test(brain-sync): give the retry-merge fixture a git identity

The retry merge commits with the ambient identity. CI and Ubicloud runners
have none under the test HOME, so the merge failed before the outgoing case
check ran and the drain reported push_failed instead of error.

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

test/brain-sync.test.ts

@@ -809,6 +809,9 @@ describe('gstack-brain-sync per-file hold', () => {
     expect(git(['-c', 'user.name=fixture', '-c', 'user.email=fixture@example.com', 'commit', '-qm', 'remote plan'], peer).status).toBe(0);
     expect(git(['push', '-q', 'origin', 'HEAD'], peer).status).toBe(0);
     expect(git(['config', 'core.ignorecase', 'false']).status).toBe(0);
+    // The retry merge commits with the ambient identity; a CI runner has none.
+    expect(git(['config', 'user.name', 'fixture']).status).toBe(0);
+    expect(git(['config', 'user.email', 'fixture@example.com']).status).toBe(0);
     write(CLEAN, '# local plan\n');
     expect(drain().status).toBe(0);
     expect(statusJson().status).toBe('error');