docs: add OpenClaw setup guide

[laolaoqi]

Closes #8

Adds a copy-paste-ready OpenClaw setup guide in docs/agents/openclaw.md with:

• Configuration instructions (YAML + env vars)
• API compatibility verification step
• Troubleshooting section

[GoDiao]

Thanks for the PR! One question before we merge: have you tested the plugins.free-way configuration shown here with a current OpenClaw build?

From a quick look at OpenClaw’s current docs, custom OpenAI-compatible endpoints seem to be configured through models.providers.<provider>.baseUrl / apiKey rather than a plugins block, so I want to avoid merging a snippet that might not work for users.

Also, could you confirm the example model? gpt-4 may not exist in a default Free-Way catalog unless a matching provider key/model mapping is available. A known Free-Way model such as llama-3.3-70b may be safer for the example.