Add provider adapters for AI-generated scenario refinement

[rushillshah]

ProbeQA currently creates deterministic scenario drafts from diffs and crawl data. Add optional model-provider adapters so users can refine drafts with OpenAI, Anthropic, or local models while keeping the generated scenario as plain Puppeteer code.\n\nAcceptance criteria:\n- Adapter interface is vendor-neutral.\n- CLI stays usable without an API key.\n- Generated scenario output remains committed source code.\n- Tests cover adapter fallback behavior.