WASHINGTON — Attorney General Merrick Garland named a special counsel on Friday to oversee the Justice Department’s investigation into the presence of classified documents at former President Donald Trump’s Florida estate as well as key aspects of a separate probe involving the Jan. 6, 2021, insurrection and efforts to undo the 2020 election.The move, announced just three days after Trump formally launched his 2024 candidacy, is a recognition of the unmistakable political implications of two …