exec-server README initialize/session lifecycle docs are stale

What happened?

codex-rs/exec-server/README.md documents initialize as returning an empty result:

{}

The current protocol returns a sessionId, and the request params also support optional resumeSessionId. The handler attaches or resumes a session and returns that session_id.

The README also says a connection close terminates processes for that client connection, but current tests cover detached sessions being resumed without killing processes.

Expected behavior

The exec-server README should document the current initialize response, resumeSessionId behavior, and session detach/resume lifecycle semantics.

Additional context

I searched open issues and PRs for terms including exec-server README, initialize, sessionId, and resumeSessionId, and did not find an existing open item that appears to cover this documentation drift.