No amendment was added. Rather, it’s implied by the Article III establishing judicial responsibilities regarding cases needing to be judged based on law, combined with the supremacy clause in Article VI establishing that the Constitution is the supreme law of the U.S.
It is an amazing series. I started from 0 and eventually ended up 100% completing all the games that are on Steam, including the Judgement games and Ishin (great game, but probably the only 100% I regret). I’m really looking forward to Gaiden and 8.