A penthouse in a historic brownstone building at 80 Commonwealth Ave. in Boston’s Back Bay is listed for sale for $6.5 million. The four-bedroom, three-bath penthouse includes a roof deck, direct access parking and direct elevator access. The property overlooks the Commonwealth Avenue Mall and is a block and a half from the Boston Public Garden. “This building boasts class and traditional elegance with a twist of modern luxury,” the listing states. Kevin Caulfield of residential brokerage…