Align UCP docs with A2A-only checkout transport

[TateLyman]

Summary

• update the architecture overview and protocol table to describe UCP checkout through POST /a2a instead of removed REST checkout endpoints
• align the PRD discovery example, UCP request headers, and checkout actions with the current A2A implementation
• update the agent guide and feature summary so future changes validate the message/send action contract

Verification

• git diff --check
• rg -n -e '/Users/[A-Za-z0-9.-]+/' -e '/home/[A-Za-z0-9.-]+/' -e '\Users[A-Za-z0-9._-]+' AGENTS.md src docs
• uv run --with pytest pytest tests/merchant/api/test_ucp_discovery.py tests/merchant/api/test_ucp_a2a.py -q