docs: document polling LLM integration status

[WH-2099]

Summary

Add a concise document for the current polling LLM integration across Graphon, plugin-sdk, plugin-daemon, and official-plugin Volcengine models.

Scope

• Describe the current start/check polling flow.
• Clarify module boundaries and daemon route contracts.
• Capture the current Seedance and Seedream behavior.
• Keep Dify adapter work out of scope for this document.

Validation

• Searched existing issues and PRs for duplicate polling LLM integration plan work.
• Documentation-only change; no runtime tests required.