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-project-row-model.test.ts

@@ -15,7 +15,6 @@ function workspace(overrides: Partial<SidebarWorkspaceEntry> = {}): SidebarWorks
     workspaceId: "/repo",
     workspaceKind: "directory",
     name: "paseo",
-    activityAt: null,
     statusBucket: "done",
     diffStat: null,
     ...overrides,
@@ -31,7 +30,6 @@ function project(overrides: Partial<SidebarProjectEntry> = {}): SidebarProjectEn
     statusBucket: "done",
     activeCount: 0,
     totalWorkspaces: 1,
-    latestActivityAt: null,
     workspaces: [workspace()],
     ...overrides,
   };

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,
   };
 }

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,
   };
 }