HCR Update from Mark Sanna: Gary M. Cohen, the official in charge of the federal health insurance marketplace, who repeatedly told Congress before its troubled rollout that it would work well, said on Thursday that he was resigning. Mr. Cohen is the chief architect of federal rules regulating the operations of private health insurance under the new health care law. He said he would leave his post to return to California at the end of this month, when the open enrollment period closes for individuals and families shopping for insurance.