Last week, StockX cofounder Josh Luber left the company he started in 2016. Luber helped build StockX into a $1 billion startup. It hasn’t gone public yet. StockX is probably in good hands. But Luber’s departure illustrates a problem that faces all startup investors — founder risk. When you invest in a startup, you’re investing in the founders and their ability to deliver. And navigating that risk is what separates average…