WASHINGTON — In taking charge of a Pentagon battered by leadership churn, the Biden administration will look to one holdover as a source of military continuity: Gen. Mark Milley, chairman of the Joint Chiefs of Staff. President-elect Joe Biden will inherit Milley as his senior military adviser, and although Biden could replace Milley, he likely won’t. A Princeton-educated history buff with the gift of gab, Milley has been a staunch defender of the military’s apolitical tradition even as President Donald Trump packed the Pentagon with political loyalists.