[Docs] Codex 集成默认要求上游实现 Responses API; MemoryProxy 会直接转发 /v1/responses,不进行翻译

`agents/codex/README.md` tells users to set `wire_api = "responses"` but never states the consequence: the upstream behind MemoryProxy must itself implement the OpenAI **Responses** API. The proxy forwards `/v1/responses` verbatim and does no protocol translation, so any OpenAI-compatible upstream that only implements `/v1/chat/completions` — which is most of them — cannot serve Codex, and the failure surfaces as a bare 404/400 from the upstream with nothing pointing at the real cause.