Arriving at his workbench, Thaddeus Mosley never knew precisely what he’d do next. Before him lay stacks of salvaged wood — discarded tree branches, the remains of a trunk. Hunks of felled cherry, walnut and sycamore, Pennsylvania hardwoods that he loved for their color and grain. All taken free from Pittsburgh tree trimmers or the city’s public works department, hauled away by Mr. Mosley in his station wagon.