test(v2): add integration tests for layout, agent-bridge, and dispatcher

Add 59 new vitest tests: layout.test.ts (30), agent-bridge.test.ts (11),
agent-dispatcher.test.ts (18). Fix unused import in sdk-messages.test.ts.
Add WebDriver E2E scaffold README. Total: 104 vitest + 29 cargo tests.

v2/tests/e2e/README.md

@@ -0,0 +1,52 @@
+# E2E Tests (WebDriver)
+
+Tauri apps use the WebDriver protocol for E2E testing (not Playwright directly).
+The app runs inside WebKit2GTK on Linux, so tests interact with the real WebView.
+
+## Prerequisites
+
+- Built Tauri app (`npm run tauri build`)
+- Display server (X11 or Wayland) -- headless Xvfb works for CI
+- `tauri-driver` installed (`cargo install tauri-driver`)
+- WebdriverIO (`npm install --save-dev @wdio/cli @wdio/local-runner @wdio/mocha-framework`)
+
+## Running
+
+```bash
+# Terminal 1: Start tauri-driver (bridges WebDriver to WebKit2GTK)
+tauri-driver
+
+# Terminal 2: Run tests
+npm run test:e2e
+```
+
+## CI setup (headless)
+
+```bash
+# Install virtual framebuffer
+sudo apt install xvfb
+
+# Run with Xvfb wrapper
+xvfb-run npm run test:e2e
+```
+
+## Writing tests
+
+Tests use WebdriverIO. Example:
+
+```typescript
+import { browser } from '@wdio/globals';
+
+describe('BTerminal', () => {
+  it('should show the terminal pane on startup', async () => {
+    const terminal = await browser.$('.terminal-pane');
+    await expect(terminal).toBeDisplayed();
+  });
+});
+```
+
+## References
+
+- Tauri WebDriver docs: https://v2.tauri.app/develop/tests/webdriver/
+- WebdriverIO docs: https://webdriver.io/
+- tauri-driver: https://crates.io/crates/tauri-driver