[good first issue] Add OpenCode setup guide

[GoDiao]

Goal

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

Required content

1. Environment variables to set:
   • OPENAI_BASE_URL=http://localhost:8787/v1
   • OPENAI_API_KEY=<your FREEWAY_API_KEY>
2. How to verify OpenCode is hitting Freeway (check Usage tab in web console)
3. Recommended models for OpenCode from Freeway's catalog

Acceptance criteria

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