ULAANBAATAR, Mongolia — South Korea and Mongolia opened a new chapter in bilateral relations Thursday, adopting a joint declaration that lays out a shared vision for a “golden age” in ties. President Lee Jae Myung and Mongolian President Ukhnaagiin Khurelsukh met for their first in-person summit on the opening day of Lee’s three-day state visit to Mongolia, marking the first visit by a South Korean president since 2011. Following the summit, the two leaders adopted a joint declaration on a “Gold