Honor authoritative room design Goal handoff

[epowell40]

Summary

• allow an exact authoritative room request to replace only an empty same-session model-expanded auto Goal
• preserve substantive and cross-session Goals
• retain the room-specific zero-interruption work budget and exact user objective

Validation

• focused room workflow: 8/8
• public backend: 940 passed, 3 established skips, 943 total
• diff check: PASS