Gate workspace_update emissions to prevent unnecessary sidebar re-renders

Server: remove activityAt computation from workspace descriptors (always
sends null for backwards compat) and deduplicate emissions in
emitWorkspaceUpdatesForWorkspaceIds using fast-deep-equal — workspace
updates are now only sent when the descriptor actually changed.

Client: remove activityAt from WorkspaceDescriptor, sidebar entry types,
and baseline sort comparators. Workspace/project ordering is fully
handled by the persisted sidebar order store.

packages/app/src/app/h/[serverId]/index.tsx

@@ -57,11 +57,6 @@ function HostIndexRouteContent() {
       );
 
       const visibleWorkspaces = sessionWorkspaces ? Array.from(sessionWorkspaces.values()) : [];
-      visibleWorkspaces.sort((left, right) => {
-        const leftTime = left.activityAt?.getTime() ?? Number.NEGATIVE_INFINITY;
-        const rightTime = right.activityAt?.getTime() ?? Number.NEGATIVE_INFINITY;
-        return rightTime - leftTime;
-      });
 
       const primaryAgent = visibleAgents[0];
       if (primaryAgent?.cwd?.trim()) {