or would it be considered an emotional trait since very few people are purposefully breeding FOR dog aggression these days?
Do you have statistics that actually say this? Or is this your personal opinion? Also I guess it depends on what you consider "aggression" is, or aggression vs. drive etc.
Anyway I think a working trait is an ingrained/bred trait that the dog does by instinct. IE: A cattle dog pushing cows around without formal training. The instinct to herd would be a working trait. However after the dog has pushed the cattle around, or even while the dog is pushing the cattle around if they get anxious, nervous, or excited in my opinion that would then be the emotional trait coming out.
I think reactivity would be considered an emotional trait. Across the board any dog can be reactive to other dogs depending on the personality of the dog. Your Pit Bull might be dog reactive but my friends Pit Bull might not be. Whereas if your Pit Bull felt the need to hold down a cow or a horse I would consider that a working trait and part of the breed. I think a working trait comes out as an inherited need to do something. Although another dog may think it is fun to bite livestock I don't think a Labrador would look at a cow or horse the same way a pit bull might because Labradors are bred for something totally different.
I just don't view reactivity as an instinctual thing. I mean, is the dog reactive because it's a pit bull? Is it reactive because it's a female? Is it reactive because it was under-socialized/over-socialized?