On a sleepy cul-de-sac amid the bucolic vineyards and grassy hills of California’s Sonoma Valley, a $4 million house has become the epicenter of a summer-long spat between angry neighbors and a new venture capital-backed startup buying up homes around the nation. The company is called Pacaso. It says it’s the fastest company in American history to achieve the “unicorn” status of a billion-dollar valuation — but its quarrels in wine country, one of the first regions where it’s begun operations, foreshadow business troubles ahead.