Seventy-six national security experts urged President-elect Donald J. Trump on Monday to reverse his hostility to the nuclear agreement signed with Iran last year and to use it as a tool to ease other tensions with the country. A report signed by the experts, including former officials from both major political parties, argued that the nuclear agreement had reduced the threat of war in the Middle East. Mr. Trump has called the nuclear agreement a foreign-policy disaster. He vowed during his campaign to renegotiate or renounce the deal, one of President Obama’s signature achievements. The...