A prominent Pentagon task force that has sought to help Afghanistan exploit its mineral wealth and expand private-sector employment is being gutted by resignations and will be forced to scale back significantly because of a congressional demand that its operations be folded into the U.S. Agency for International Development. The director of the task force, Deputy Undersecretary of Defense Paul A. Brinkley, has decided to quit on June 30, a move that has prompted several key members of his 100-person team to announce their departures as well. The exodus has alarmed senior U.S. military officials, including Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, and Gen. David H. Petraeus, the top U.S. commander in Afghanistan, who view the group’s job-creation efforts as an important component of the overall U.S. counterinsurgency mission.