Make the workspace right pane a first-class tab host (#3287)

* feat(app): add persistent workspace side panes

Keep the common review and file-browsing workflow beside the active agent without tearing down pane contents. Workspace tabs now share one compact chip language, assistant file links route into the retained right pane, and diff/file trees share stable geometry.

* fix(app): localize pull request empty state

* test(app): update explorer pane coverage

* fix(app): preserve explorer pane workflows

* test(app): make explorer tab selection idempotent

* fix(app): persist workspace review pins

* fix(app): preserve sidebar scrim drag hook

* fix(app): restore companion pane interactions

* test(app): follow selected sidebar surface

packages/app/e2e/browser/workspace-navigation-regression.spec.ts

@@ -79,6 +79,12 @@ async function getVisibleDraftTabCount(page: Page): Promise<number> {
 }
 
 async function closeFirstVisibleDraftTab(page: Page): Promise<void> {
+  const tab = page
+    .locator('[data-testid^="workspace-tab-draft"]')
+    .filter({ visible: true })
+    .first();
+  await expect(tab).toBeVisible({ timeout: 30_000 });
+  await tab.hover();
   const closeButton = page.locator('[data-testid^="workspace-draft-close-"]').filter({
     visible: true,
   });