[good first issue] Add OpenClaw setup guide

[GoDiao]

Goal

Add a copy-paste-ready setup guide for OpenClaw in the README (or docs/agents/openclaw.md).

Required content

1. How OpenClaw accepts custom base URL and API key
2. Values: http://localhost:8787/v1 + FREEWAY_API_KEY
3. Verification step

Acceptance criteria

• A new user can follow the guide and have OpenClaw 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.