I used to drag the agent.txt in the chat,as you described. But according to your estimate it takes 160k tokens, and posting it in chat, directly reaches chat limit,just with my prompt. I don't even use tile maps yet just a sprite for placeholder background.
But posting the system-agent in the behavior of Claude and then attaching the master prompt.txt as a project file. The master prompt only takes 9% of project size and I can have a fairly long conversation without hitting the limit. And your implementation is fully functional.
So nothing to do from your side, if used correctly (I'm a dummy) Claude fully supports this solution.