Processes surrounding claude context files

I left [a comment](https://github.com/ethereum/execution-specs/pull/2024#issuecomment-3837605914):

> - Is there any way we can put these files under a dot directory, or hide them from us humans in some other way?
> - What are the expectations for non-claude users when modifying the repository?

I think it's important to document (perhaps in `CONTRIBUTING.md`) what the expectations are for updating these files.

Perhaps even more important, is there some metric we can use for measuring how accurate the files are? I'm thinking an Actions workflow that compares the context files to the source and fails if they diverge too much.