Even as the Bush administration works to relax clean-air regulations on coal-fired power plants, New Jersey’s biggest energy supplier agreed yesterday to spend $337 million over the next 10 years to cut emissions from two plants. U.S. Attorney General John Ashcroft said the settlement between PSEG Power showed a “continuing commitment to enforce vigorously the Clean Air Act.” The Clinton administration sued PSEG Power and several other utilities for violating new source review regulations that require owners of older power plants to update pollution-control systems when making other significant improvements to their plants.