exec-server README omits http/request protocol surface

What happened?

codex-rs/exec-server/README.md documents process and filesystem RPCs, but it does not appear to document the http/request API surface.

The current protocol defines:

• http/request
• http/request/bodyDelta

The server registry registers http/request, and integration tests cover an exec-server websocket http/request performing a real HTTP request and returning the response body.

Expected behavior

The exec-server README should document the http/request request and related http/request/bodyDelta notification so clients can discover the supported HTTP protocol surface.

Additional context

I searched open issues and PRs for terms including exec-server README, http/request, and bodyDelta, and did not find an existing open item that appears to cover this README gap.