fix(app): land on agent tab after creating a workspace

Workspace creation landed on the setup tab because openTab implicitly
focused the tab it opened, and the setup auto-open effect ran after
the setup dialog's navigation, stomping on the agent tab's focus.

Split the store API into explicit openTabFocused and openTabInBackground
so every caller declares intent. The setup auto-open effect now opens in
background and no longer steals focus; user-intent call sites (sidebar,
kebab menus, file explorer, terminal creation, prepareWorkspaceTab)
use openTabFocused and drop the redundant paired focusTab call.

packages/app/src/utils/workspace-navigation.ts

@@ -33,11 +33,7 @@ export function prepareWorkspaceTab(input: PrepareWorkspaceTabInput) {
       workspaceId: input.workspaceId,
     }) ?? "";
 
-  const tabId = useWorkspaceLayoutStore.getState().openTab(key, target);
-
-  if (tabId) {
-    useWorkspaceLayoutStore.getState().focusTab(key, tabId);
-  }
+  useWorkspaceLayoutStore.getState().openTabFocused(key, target);
 
   if (input.pin && target.kind === "agent") {
     useWorkspaceLayoutStore.getState().pinAgent(key, target.agentId);