BELFAST, Maine — In the last few weeks, Margaret Cunningham has harvested the potatoes from her vegetable garden, painted her porch, driven herself to church, started a quilting class and put up quarts of tomatoes and pints of raspberry, blackberry and blueberry jams and jellies. It may not sound extraordinary. But when you learn that Cunningham is 100 years old, suddenly the list takes on a deeper resonance. “I like to keep busy,” she said while sitting in her favorite chair at her tidy, cozy Belfast home.