Credit the user who forked a task as a co-author on its merge

internal/agents/manager_test.go

@@ -1148,6 +1148,15 @@ func TestForkSnapshot(t *testing.T) {
 		t.Fatalf("fork session %+v", sess)
 	}
 
+	coAuthors, err := m.CoAuthors(e.ctx, fork.Id, "carol@example.com")
+	if err != nil || len(coAuthors) != 1 || coAuthors[0].Login != login {
+		t.Fatalf("fork co-authors %+v %v, want the forker %s", coAuthors, err, login)
+	}
+	coAuthors, err = m.CoAuthors(e.ctx, e.task.Id, "carol@example.com")
+	if err != nil || len(coAuthors) != 1 || coAuthors[0].Login != login {
+		t.Fatalf("source co-authors %+v %v", coAuthors, err)
+	}
+
 	_, err = m.Send(e.ctx, sess.Id, login, "Without using any tools, list the contents of every file you have created in this conversation, one per line, and nothing else.", "")
 	if err != nil {
 		t.Fatalf("send to fork: %v", err)

internal/store/store.go

@@ -795,7 +795,7 @@ func (s *Store) Participants(ctx context.Context, taskID string) ([]api.Particip
 	rows, err := s.db.QueryContext(ctx, `
 		SELECT u.login, u.display_name, u.git_name, u.git_email
 		FROM events e JOIN users u ON u.login = e.actor
-		WHERE e.task_id = ? AND e.kind = 'user.message'
+		WHERE e.task_id = ? AND (e.kind = 'user.message' OR (e.kind = 'task.created' AND json_extract(e.payload, '$.forked_from') IS NOT NULL))
 		GROUP BY u.login ORDER BY min(e.seq)
 	`, taskID)
 	if err != nil {