Adds room transcript context compaction (FN-4180) to the dashboard chat layer — widens the fetch window then compacts older messages into a summary window to reduce token overhead while keeping full context recent. Includes a new 127-line test suite covering the compaction logic, a small fix to reus
Fusion-Task-Id: FN-4180
Fusion-Task-Lineage: 9e7e3a32-f2da-4c86-871b-37891f747561