feat: open Changes as a workspace tab (#2298)

Open the complete working comparison in a desktop workspace tab while preserving inline sidebar diffs when the tab is closed.

Working and commit comparisons share one diff panel, and workspace layout is the sole owner of live tab persistence and identity.

Refs #1520

packages/app/src/screens/workspace/workspace-tab-menu.test.ts

@@ -1,5 +1,8 @@
 import { describe, expect, it, vi } from "vitest";
-import { buildWorkspaceTabMenuEntries } from "@/screens/workspace/workspace-tab-menu";
+import {
+  buildWorkspaceDesktopTabActions,
+  buildWorkspaceTabMenuEntries,
+} from "@/screens/workspace/workspace-tab-menu";
 import type { WorkspaceTabDescriptor } from "@/screens/workspace/workspace-tabs-types";
 
 function createAgentTab(): WorkspaceTabDescriptor {
@@ -253,6 +256,37 @@ describe("buildWorkspaceTabMenuEntries", () => {
     expect(onCopyFilePath).toHaveBeenCalledWith("/some/path.ts");
   });
 
+  it("uses a Changes close id for the working diff tab", () => {
+    const actions = buildWorkspaceDesktopTabActions({
+      tab: {
+        key: "working_diff_abc",
+        tabId: "working_diff_abc",
+        kind: "working_diff",
+        target: {
+          kind: "working_diff",
+          focusPath: "src/example.ts",
+          focusRequestId: 1,
+        },
+      },
+      index: 0,
+      tabCount: 1,
+      onCopyResumeCommand: vi.fn(),
+      onCopyAgentId: vi.fn(),
+      onCopyFilePath: vi.fn(),
+      onReloadAgent: vi.fn(),
+      onRenameTab: vi.fn(),
+      onCloseTab: vi.fn(),
+      onCloseTabsToLeft: vi.fn(),
+      onCloseTabsToRight: vi.fn(),
+      onCloseOtherTabs: vi.fn(),
+    });
+
+    expect(actions.closeButtonTestId).toMatch(/^workspace-working-diff-close-/);
+    expect(actions.menuEntries).not.toContainEqual(
+      expect.objectContaining({ kind: "item", key: "copy-file-path" }),
+    );
+  });
+
   it("uses the same rename entry shape for agent and terminal tabs", () => {
     const terminalTab: WorkspaceTabDescriptor = {
       key: "terminal_abc",