Out-of-scope requests are refused gracefully

A domain-scoped assistant without a scope boundary becomes a general-purpose chatbot the moment a user asks anything off-topic. Customer-support bots start giving medical advice, code assistants opine on tax law, and the company inherits liability for confabulated answers outside its expertise. This violates the inference-contract taxon — you shipped a support product and delivered an unconstrained LLM. Worse, out-of-scope answers are where hallucination rates spike, because the model has no grounding.

Write an explicit scope statement in the system prompt naming the domain, and define a graceful refusal template that redirects users rather than terse-rejecting them. Update lib/ai/prompts.ts with:

const systemPrompt = `You are a support assistant for [Product]. You answer installation, configuration, billing, and usage questions. For anything else, reply: "That's outside what I can help with here. For [topic], try [resource]."`