In a treehouse nestled in redwoods north of San Francisco, Gov. Gavin Newsom stood cold and hungry as Sergey Brin, the world’s fourth-richest man, and his wellness-influencer girlfriend told him they were leaving the state. It was late in the evening at a Christmas party hosted by crypto titan Chris Larsen — featuring singer Janelle Monáe and a towering abominable snowman with glowing red eyes — when Brin and his partner, Gerelyn Gilbert-Soto, confronted Newsom about a new proposal to tax billionaires in California, according to people who’ve spoken with the governor. Such a levy could hit Brin’s stake in Alphabet Inc. and his $272.6-billion fortune. Newsom, who opposes the wealth tax, was still telling people about the lengthy exchange at the party months later, complaining of a lingering cold the pair had given him, according to the people, who asked not to be named discussing private conversations with the governor. Brin, meanwhile, followed through. He left the state, bought a lakeside mansion in Nevada, and started bankrolling a billionaire political uprising in California. Newsom, …