Q142: a conversation is read only up to the message the reader actually saw

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/shared/commands/chat.ts

@@ -38,4 +38,6 @@ export interface MarkChatSpaceReadInput {
   orgId: string;
   employeeId: string;
   spaceId: string;
+  /** The newest message the reader actually saw; later arrivals stay unread. */
+  upToMessageId: string;
 }