[good first issue] Add Claude Code setup guide

[GoDiao]

Goal

Add a copy-paste-ready setup guide for Claude Code in the README (or docs/agents/claude-code.md).

Required content

1. Environment variables to set:
   • ANTHROPIC_BASE_URL=http://localhost:8787
   • ANTHROPIC_API_KEY=<your FREEWAY_API_KEY or any non-empty string>
2. Launch command after setting env vars
3. One verification step (e.g., run a test prompt and confirm it routes through Freeway)
4. Optional: mention that Freeway auto-routes to the best available free provider

Acceptance criteria

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

Labels

documentation, good first issue, help wanted