Jack Smith, the special counsel who pursued two federal prosecutions of Donald J. Trump, plans to finish his work and resign along with other members of his team before Mr. Trump takes office in January, people familiar with his plans said. Mr. Smith’s goal, they said, is to not leave any significant part of his work for others to complete and to get ahead of the president-elect’s promise to fire him within “two seconds” of being sworn in. Mr. Smith, who since taking office two...