Harvey cofounder Gabe Pereyra.HarveyLegal tech giant Harvey has announced its first in-house, proprietary AI model, Tenet.The model is designed to handle legal work typically done by lawyers over many hours, at lower cost.The move clears a path for Harvey to someday help law firms train custom models on their data.Harvey built an $11 billion legal-software business on top of other companies' AI models. Now it's trying to prove it can build one o…