That's a good idea to give more positive instructions as opposed to negative instructions. I think you've stated it well, I suppose the problem with negative instructions is that the LLM doesn't know what to do instead.

"Never re-write a helper function" vs "Always search for helper functions before writing one" the "never... " one doesn't tell the LLM what to do, so it would have to make the logical leap from not re-writing to knowing that it should search. While it's a minor leap to make in isolation, I suppose stacking many negative rules in an AGENTS.md would assume that every time it will always make that logical conclusion on what to do.