Elon Musk subpoenaed Jack Dorsey, co-founder of Twitter Inc. and his long-time friend, in his defence against the social media company’s lawsuit to make him complete his proposed US$44 billion buyout. Dorsey, who stepped down as Twitter’s chief executive officer last year, has been an energetic booster of Musk’s bid for the company, tweeting in April that Musk was the “singular solution I trust” to take over. The subpoena follows a pair filed on Friday for Kayvon Beykpour, former head of consumer product at Twitter, and Bruce Falck, formerly in charge of revenue product. The billionaire is rapidly marshaling documents…