>lift the weight for every shot, which requires energy.

All harvesting requires some secondary-energy source.

Gravity is a resource, it takes effort to convert it to a form that can be applied - just like wheat, photons, air guns, etc.

> It's not obvious whether a trebuchet is better than compressed air, like a BB gun.

True, it’s not obvious yet, but it sure is fun following along and imagining the final, highly optimal version making its way through the process. I’d use it for some seed bombing hijinks, anyway. Green up the place, I would.