A member of the far-right Oath Keepers militia group that stormed the U.S. Capitol on Jan. 6 has pleaded guilty and will cooperate with investigators against his fellow extremists, marking another win for the Justice Department in its major conspiracy case stemming from the attack. Jason Dolan, 45, is the fourth person associated with the group that recruits current and former military, police and first responders to plead guilty to conspiracy charges for their roles in Capitol riot. The former Marine from Wellington …