Simplify upsell stack and align tests with current prompt wiring.

Raise upsell skill: analytical buyer mode, answer-first, structured closing.

Enforce brief sales-focused chatbot rules with spoiler guard and buy CTAs.

Restructure chat flow: hello shows clickable book list, then engaging Q&A.

Fix: replace all stale app.services.retriever imports with vector_store

Restructure project for HF Spaces deployment