The F.B.I. director, James B. Comey, told his top agents from around the country that he had been asked by President Trump to stay on the job running the federal government’s top law enforcement agency, according to people familiar with the matter. A decision to retain Mr. Comey would spare the president another potentially bruising confirmation battle. It also would keep Mr. Comey at the center of the F.B.I.’s investigation into several Trump associates and their potential ties with the Russian government.