Elon Musk presented a prototype at an artificial-intelligence event that unfolded late Friday in Palo Alto, California. Customers should be able to receive the robot, once ordered, in three to five years, Mr. Musk said. It is expected to cost much less than a car, with a price point below $20,000. ‘It will, I think, turn the whole notion of what’s an economy on its head, at the point at which you have no shortage of labor,’ Musk said Aug. 4 at Tesla’s annual shareholder meeting. He has previously expressed concerns about how such technology could be used. The battery-powered robot should be. able to handle difficult chores, Tesla said, including lifting a half-ton, nine-foot concert grand piano. . . .