Issue 008: Real AI chat API boundary

[cody257]

Summary

• Adds /api/chat as a Vercel Function that keeps model API keys server-side.
• Adds a provider boundary for OpenAI Responses API and Anthropic Messages API.
• Adds prompt creation from chat messages using the negotiation context engine and prompt playbooks.
• Adds a browser chat client toggle: default mock, optional VITE_HAGGLY_CHAT_MODE=api.
• Adds .env.example and Vercel function packaging for prompt playbooks.

Test Plan

• npm run lint
• npm test
• npm run test:e2e
• npm run build

Notes

• No secrets committed.
• No auth, billing, database, rate limiter, or public-traffic hardening included.
• Local mock mode remains the default so existing tests and previews do not need API keys.
• Real model mode requires server env vars plus VITE_HAGGLY_CHAT_MODE=api.

API docs checked

• OpenAI Responses API: https://developers.openai.com/api/docs/guides/text
• Anthropic Messages API: https://platform.claude.com/docs/en/build-with-claude/working-with-messages