I suspect it won't be a harness, but just a more specific LLM trained in the universe of user-selected context of vetted resources.

Why? Because LLMs are always going to be dumb when they're trained at scale. Their ability to speak software diverges from their friendly user input layer. A harness won't overcome that, but an LLM saddle ontop of a larger model would provide the type of feedback loops you'd want to look into.

I don't think you'll find two deterministic systems will produce much.