By Ross Kerber BOSTON (Reuters) -New York’s state pension fund will sell $238 million worth of stock and debt it holds across 21 shale oil and gas companies including Chesapeake Energy Corp, Hess Corp and Pioneer Natural Resources, saying they have not shown they are ready to move to a low-emissions economy. However, the fund will keep another 21 shale companies including ConocoPhillips, CNX Resources Corp and EQT Corp according to material reviewed by Reuters from New York Comptroller Thomas DiNapoli, who oversees retirement assets. “To protect the state pension fund, we are restricting inves…