“The usual rule in post-conviction proceedings is that a lawyer’s mistakes are imputed to the client, on the theory that the lawyer is the client’s agent. ‘We do not disturb that general rule,’ Justice Ginsburg wrote. What was different here, she said, was that the lawyers had abandoned their client, severing the agency relationship. ‘Moreover,’ Justice Ginsburg said in announcing the decision from the bench Wednesday morning, Mr. Maples ‘lacked any clue that he had better fend for himself.’”