MEMPHIS, Tenn. — Jimmy Carter resumed his role as Habitat for Humanity’s most prominent booster on Monday, donning a white hard hat and a worn leather belt stocked with his own tools to hammer and saw with other volunteers building a home. “We haven’t cut back on my schedule yet,” Carter said, seeming invigorated during an Associated Press interview. The former president celebrated his 91st birthday in October, and is undergoing treatment on cancer found in his liver and brain.