Twitter is Elon Musk’s. The world’s richest person reportedly closed Thursday on his deal to buy the social-networking giant for $54.20 per share, an inflated price he offered in April before taking the company on a ride whose destination didn’t even start to become clear until a few days ago. Anyone could be forgiven for skepticism that the ordeal has really ended, given Musk’s penchant for changing his mind. But it is actually over now, according to the Washington Post, which said Musk wrapped up the deal and then fired CEO Parag Agrawal and several other top executives as he...