Add negotiation capability mentions to doc pages

[atc964]

Summary

• Adds negotiation capability cross-references to 10 documentation pages where it was missing
• Each page gets brief, contextual mentions (a sentence, table row, admonition, or small subsection) linking to the existing Negotiation Guide
• No page rewrites; additions only

Pages Updated (priority order)

1. Seller Agent Integration — updated opening sentence, added Negotiation subsection, added negotiation step to sequence diagram, added Related link
2. Booking Flow — added negotiation step to sequence diagram, added note admonition about negotiation between research and booking
3. Bookings API — added info admonition about negotiation during running phase and kpis.target_cpm driving negotiation
4. Quickstart — added Negotiation Guide link to Next Steps
5. Architecture Overview — added NegotiationClient to Component Summary table, added "negotiates pricing" to closing description
6. OpenDirect Integration — added note that negotiation happens via A2A/NegotiationClient not OpenDirect, added Reference link
7. Agent Hierarchy — added negotiation step to execution flow sequence diagram
8. Products API — added tip admonition about negotiating before booking after product search
9. Data Models — added Negotiation Models section with cross-reference to negotiation guide
10. Seller Discovery — added "Negotiate" step to discovery workflow diagram

Test plan

• mkdocs build --strict passes with zero warnings/errors
• All relative links point to existing ../guides/negotiation.md
• No changes to "Coming Soon" stub pages (state-machine.md, event-bus.md)

bead: buyer-eeq

🤖 Generated with Claude Code