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/workspace-archive-navigation.test.ts

@@ -17,7 +17,6 @@ function workspace(
     workspaceKind: input.workspaceKind ?? "worktree",
     name: input.name ?? input.id,
     status: input.status ?? "done",
-    activityAt: input.activityAt ?? null,
     diffStat: input.diffStat ?? null,
   };
 }