Rotisserie Chicken with Yogurt Sauce and Herb Relish

1 cup Greek yogurt or labneh

3 inner celery stalks, sliced (¾ cup), plus ¼ cup leaves, chopped

5 tablespoons extra-virgin olive oil

¼ teaspoon ground cumin

Kosher salt and freshly ground pepper

1 jalapeño, finely chopped (¼ cup), seeds removed for less heat if desired

½ cup finely chopped fresh cilantro leaves and tender stems

⅛ teaspoon ground cinnamon

1 tablespoon distilled white vinegar

½ cup thinly sliced radishes (from about 3)

1 store-bought rotisserie chicken

Pitas and store-bought hummus, for serving