WASHINGTON — The Obama administration has written 275 briefing papers for the incoming Trump administration: nearly 1,000 pages of classified material on North Korea’s nuclear program, the military campaign against the Islamic State, tensions in the South China Sea, and every other kind of threat the new team could face in its first weeks in office. Nobody in the current administration knows whether anyone in the next has read any of it. Less than three days before President Obama turns the keys to the White House, and the nuclear codes, over to President-elect Donald J. Trump, Mr. Trump’s transition staff...