Get cart instructions text.

Compiles a saved cart's full findings payload into the agent-ready <instruction_for_agents> XML block. Each finding includes repo/PR provenance, reviewer facet and agent, the finding body as CDATA, and the optional per-item annotation as a note attribute. Uses the same builder as the get_cart_findings MCP tool. Only returns text for saved (server-persisted) carts. The caller is expected to paste this text into a local agent chat or pass it to an MCP session.