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/utils/sidebar-shortcuts.test.ts

@@ -13,7 +13,6 @@ function workspace(serverId: string, cwd: string): SidebarWorkspaceEntry {
     workspaceId: cwd,
     workspaceKind: "local_checkout",
     name: cwd,
-    activityAt: null,
     statusBucket: "done",
     diffStat: null,
   };
@@ -28,7 +27,6 @@ function project(projectKey: string, workspaces: SidebarWorkspaceEntry[]): Sideb
     statusBucket: "done",
     activeCount: 0,
     totalWorkspaces: workspaces.length,
-    latestActivityAt: null,
     workspaces,
   };
 }