Cookie stuffing already sent two men to federal prison. Shawn Hogan made $28 million as eBay's #1 affiliate before the FBI raided his home. Brian Dunning, the #2, took in $5.2 million. Both pleaded guilty to wire fraud. Now leaked Slack messages show Phoebe Gates' startup ran the same playbook, and the founders knew. Here's what Phia's extension did. During checkout it opened a background tab and inserted its own referral code, claiming commission on sales it never drove, sometimes overriding…