[good first issue] Add Codex CLI setup guide

[GoDiao]

Goal

Add a copy-paste-ready setup guide for OpenAI Codex CLI in the README (or docs/agents/codex.md).

Required content

1. Environment variables to set:
   • OPENAI_BASE_URL=http://localhost:8787/v1
   • OPENAI_API_KEY=<your FREEWAY_API_KEY>
2. How Codex CLI picks models and how to map to Freeway model IDs
3. Verification step

Acceptance criteria

• A new user can follow the guide and have Codex CLI talking to Freeway in under 2 minutes
• The guide is linked from the main README

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.