Fix Codex upstream compatibility and docs

[ppop123]

Summary

• normalize Codex requests with required instructions/store semantics and parse Codex SSE responses for non-stream calls
• keep /v1/responses and /v1/chat/completions compatible with live Codex upstream while preserving streaming behavior
• update README and README_CN with dual-provider quick start, routing semantics, Docker notes, and admin status fields

Test Plan

• npx tsx --test tests/*.test.ts
• npm run build
• manual smoke test against real local Codex and Claude sessions