Built-in commenting is planned as part of the Memberships & Subscriptions feature. Ghost needs a robust visitor authentication system for native commenting to work well and will involve a big concept-shift for Ghost’s front-end from static generation to each page load potentially being dynamic.
In the meantime, Discourse and Talk are great options. If you look at them you can see the level of effort that is required to do commenting, moderation, and spam protection at scale.