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/screens/workspace/workspace-source-of-truth.test.ts

@@ -17,7 +17,6 @@ describe("workspace source of truth consumption", () => {
       workspaceKind: "local_checkout",
       name: "feat/workspace-sot",
       status: "running",
-      activityAt: new Date("2026-03-01T00:00:00.000Z"),
       diffStat: null,
     };