A new sustainability index is poised to take the guesswork out of how green your garments are, doing for clothing and shoes what the federal Energy Star rating currently does for electronics and appliances. For the past three years, a group of 100 major apparel brands and retailers — including Levi Strauss, Nike, Patagonia, Timberland, and Target — have been hard at work on Eco Index, a software tool that measures the ecological and human-rights impacts of their products from cradle to grave.