Issue 006: Negotiation context engine

[cody257]

Summary

• Adds a pure negotiation context engine for offer classification, counter-offer math, formatted prices, missing fields, and next-move labels.
• Reuses the same classification/counter logic inside the legacy deterministic message generator.
• Adds unit coverage for lowball, counter, close, acceptable, accept, counter math, formatting, and incomplete buyer context.

Test Plan

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

Notes

• No API calls, secrets, auth, database, or provider wiring included.
• This is the prep layer for prompt assembly and the hosted model endpoint.