The flat-messages refactor left ChatThreadPanel orphaned: ChatPage passed a
no-op onOpenThread and ChatTimeline hardcoded onOpenThread={null}, so threads
(including /escalate board flows) were unreachable from the UI and the
desktop e2e failed on main. Reconnect open/close/reply/board-intervention
wiring around the flat timeline, show reply counts on thread roots, and
clear the lint debt that severed feature left behind.
ZacharyZhang-NY