Senior Bush administration officials said publicly for the first time yesterday that the United States is set to shoot down any North Korean missile launch that threatens the United States. National Security Adviser Stephen J. Hadley, briefing reporters during President Bush's brief visit here, said the United States has a missile defense system with "limited operational capability" that could be used to try to shoot down an incoming North Korean missile, but he added that U.S. officials were vigorously pursuing a diplomatic push to head off a test launch by Pyongyang. "The purpose, of course, of that missile defense system...