IME a dedicated testing / QA agent sounds nice but doesn't work, for same reasons as AI / human interaction. The more you try to diverge from the original dev agent's approach, the less and less chance there is that the dev agent will get to where you want it to be. Far more frequently it'll get stuck in a loop between two options that are both not what you want.

So adding a QA agent, while it sounds logical, just ends up being even more of this. Rather than converging on a solution, they just get all out of whack. Until that is solved, far better just to have your dev agent be smart about doing its own QA.

The only way I could see the QA agent idea working now is if it had the power to roll back the entire change, reset the dev agent, update the task with some hints of things not to overlook, and trigger the dev process from scratch. But that seems pretty inefficient, and IDK if it would work any better.