Agriculture includes planting the plants, not just harvesting and eating them. If you're just picking and eating stuff, that's not agriculture, it's gathering.
And you can't discover planting plants, you have to do that on purpose.
I read someone thought maybe people eating certain seeds would take a crap outside and then they noticed plants growing where they crapped, but that's not discovery of the actual planting of seeds. Just information allowing for the invention. Much like observing that things can roll contributed to inventing the wheel.
As I said on another thread, the greatest invention of them all is the spear. Without the spear there would be no agriculture, no wheel, no nothing. Just giant cats, wolves and bears eating everyone.