Keep large sidebars responsive (#1966)

* perf(app): keep large sidebars responsive

Sidebar rows rescanned every agent for each session update, multiplying work with both workspace and agent count. Build workspace activity once and share the derived sidebar model so updates scale with the underlying collections.

* test(app): use valid agent fixture for workspace restore

* refactor(app): keep sidebar model surface focused

packages/app/src/hooks/sidebar-workspaces-view-model.test.ts

@@ -231,6 +231,7 @@ describe("shared sidebar workspace model", () => {
       sessions: [
         {
           serverId: "host-a",
+          workspaceAgentActivity: new Map(),
           workspaces: new Map([
             [
               "main",
@@ -246,6 +247,7 @@ describe("shared sidebar workspace model", () => {
         },
         {
           serverId: "host-b",
+          workspaceAgentActivity: new Map(),
           workspaces: new Map([
             [
               "feature",