You're going to include specific coding style rules in your README? Or other really agent-specific things like guidance about spawning sub-agents?
They are separate for a good reason. My CLAUDE.md and README.md look very different.
README often contains only basic context for the project and instructions for basic tasks like running it and building it from source. If additional information for developers, like coding conventions, is short enough compared to the rest of the README then it sometimes gets added there too, but if there's a lot of it then it's frequently kept elsewhere to prevent README from getting overwhelming for end users and random people just checking out the project.
I don't think anything requires a README.md to be monolithic. They often provide the introductory material that you mention here, then link out to other appropriate files for contribution guidelines, etc.
It should not contain personal preference. It should contain project conventions.
Project guidelines, how to build your project, where to find or implement different types of features, are not personal preference. If different members of your team disagree on these things, that is a problem.
To share the most effective workflows so people don't have to muddle around figuring out what to do?
You're going to try to tell people how to code with agents in the readme? Why?
Why would you publish agent specific things to your codebase? That's personal preference and doesn't have anything to do with the project.