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/add-changed-file-to-chat.spec.ts

@@ -2,6 +2,7 @@ import { mkdir, writeFile } from "node:fs/promises";
 import path from "node:path";
 import { test, expect, type Page } from "../support/fixtures";
 import { seedMockAgentWorkspace, openAgentRoute } from "../support/helpers/mock-agent";
+import { openChangesPanel } from "../support/helpers/workspace-tabs";
 
 function visibleComposer(page: Page) {
   return page.locator("textarea[data-composer-input]").filter({ visible: true }).first();
@@ -31,8 +32,7 @@ test("adds a changed file to the focused chat without replacing its composer dra
     await expect(agentComposer).toBeEditable({ timeout: 30_000 });
     await agentComposer.fill("Preserve this thought");
 
-    await page.getByRole("button", { name: "Open explorer" }).click();
-    await page.getByTestId("explorer-tab-changes").click();
+    await openChangesPanel(page);
     const changedFile = page.getByText("changed file.ts", { exact: true }).first();
     await expect(changedFile).toBeVisible({ timeout: 30_000 });
     await page.getByTestId("diff-file-0-toggle").click({ button: "right" });