cmd/sgai/skel: relay retrospective questions literally

[ucirello]

Summary

• preserve the exact post-prefix RETRO_QUESTION body when the coordinator relays retrospective prompts to the human partner
• derive retrospective approval choices from the selectable bullet list and treat [MULTI-SELECT] in the prefix as the only source of multiSelect=true
• update the retrospective prompt so its outbound questions are self-contained and compatible with the coordinator's literal relay contract

Test Plan

• make absorb-sgai