How to organize code like you said, and how agents interact with it, to keep the actual context window small is the fundamental challenge.
I looked at that response by GP (rgbrenner) and refrained from replying because if someone is both running hundreds of agents at a time AND oblivious to what "context window" means, there is no possible sane discourse that would result from any engagement.
Doesn't change my point: the amount of code the agent can operate on is very large, if not unlimited, as long as you put even a little bit of thought into structuring things so it can be divided along a boundary.
If you let the codebase degrade into spaghetti, then the LLM is going to have the same problem any engineer would have with that. The rules for good code didn't disappear.
It's like like if your context window with one agent is n, your context window with 10 agents is n/10. It is some skill, but that is also where a lot of the advances are coming in.