Document usage of existing Chat record with Agent

[sarrietav-dev]

Added instructions for using an existing Chat record with Agent.

What this does

Adds a section to the Agents docs about how to instantiate an Agent using an existing chat. I found this useful when handling authorization of chats/agents:

chat_record = Current.user.chats.find(params[:id])
@chat = WorkAssistant.new(chat: chat_record)

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case