President-elect Donald J. Trump’s transition operation announced on Tuesday that it had belatedly signed an agreement with the Justice Department that will allow the F.B.I. to conduct background checks on people Mr. Trump intends to appoint as senior officials in his new administration.F.B.I. background checks have long been a routine part of transitions. Law-enforcement vetting of a president-elect’s senior team is normally part of decisions about whether they can be entrusted with access to national security secrets, and senators traditionally want to see such dossiers during the confirmation process.But Mr. Trump, who is hostile to the F.B.I. because of its...