Power lifting and Olympic lifting will serve you best. They will give you strength and explosiveness which are two main components of agility. Unless your definition is different than mine.
If you want to be able to change directions in a hurry, you need to have very strong legs, and if you need to get up to full speed in a hurry after that change in directions, they need to be explosively strong.
Squats, deadlifts, powercleans, hangcleans, all of these are your friend. I don't think anything else will help your rugby game more than this, other than actually practicing/playing rugby.
Vegetables are what food eats.