fix(app): seed changes tab in explorer pane even when setup tab occupies it

The explorer companion pane retention commit (de9e85862) auto-opens the
setup progress tab into the hidden explorer pane. toggleWorkspaceExplorerPane
only seeded the working_diff (Changes) tab when the pane was completely
empty, so once the setup tab landed there, clicking the explorer toggle
could never surface Changes/Files again via that pane's default tab.

Also fixes two e2e testid locators (workspace-new-tab-menu-trigger) and
one unit test assertion (browser automation handler) that assumed a bare
pane root, which predates that same explorer-pane-retention change: the
workspace root is now always a group with a hidden companion pane, so
unscoped locators/assertions must target the visible/named pane.

packages/app/src/screens/workspace/workspace-screen.tsx

@@ -1404,7 +1404,15 @@ function toggleWorkspaceExplorerPane(input: ToggleWorkspaceExplorerPaneInput): v
       store.hidePane(input.persistenceKey, explorerPane.id);
       return;
     }
-    if (explorerPane.tabIds.length === 0) {
+    // The companion pane can already hold an unrelated tab (the setup
+    // progress tab auto-opens here) before the user ever toggles it, so
+    // "empty" is the wrong signal for "needs a Changes tab seeded". Check
+    // for the working_diff tab specifically.
+    const layoutTabs = collectAllTabs(layout.root);
+    const hasWorkingDiffTab = explorerPane.tabIds.some(
+      (tabId) => layoutTabs.find((tab) => tab.tabId === tabId)?.target.kind === "working_diff",
+    );
+    if (!hasWorkingDiffTab) {
       const tabId = parentTabId
         ? store.openChildTabFocused(input.persistenceKey, { kind: "working_diff" }, parentTabId)
         : store.openTabFocused(input.persistenceKey, { kind: "working_diff" });