Secretary of State John F. Kerry joined high-level talks Sunday in Geneva that could curtail Iran’s nuclear program, as negotiators race to reach a comprehensive agreement by the end of next month.Kerry met with his Iranian counterpart, Foreign Minister Mohammad Javad Zarif, in the lakeside city. In an indication that the talks are at a sensitive stage, they were joined by Ernest Moniz, the secretary of energy, and Iran’s nuclear chief, Ali Akbar Salehi.