In a series of tweets on Monday, Mr. Musk accused Apple of threatening to pull Twitter from its App Store and of trying to censor. SAN FRANCISCO — “What’s going on here @tim_cook?” Elon Musk tweeted on Monday to Tim Cook, the chief executive of Apple, igniting a spat between the world’s richest man and the world’s most valuable public company. In a series of tweets over 15 minutes, Mr. Musk, the new owner of Twitter, accused Apple of threatening to withhold Twitter from its App Store, a move that would limit some new users from downloading the app. The...