feat: unified E2E testing engine — 205 tests, dual-stack support

Infrastructure:
- adapters/: base, tauri (port 9750), electrobun (port 9761 + PTY daemon)
- helpers/: 120+ centralized selectors, reusable actions, custom assertions
- wdio.shared.conf.js + stack-specific configs

18 unified specs (205 tests):
splash(6) smoke(15) settings(19) terminal(14) agent(15) search(12)
files(15) comms(10) tasks(10) theme(12) groups(12) keyboard(8)
notifications(10) diagnostics(8) status-bar(12) context(9)
worktree(8) llm-judged(10)

Daemon: --stack tauri|electrobun|both flag
Scripts: test:e2e:tauri, test:e2e:electrobun, test:e2e:both

tests/e2e/adapters/base-adapter.ts

@@ -0,0 +1,43 @@
+/**
+ * Abstract stack adapter — defines the lifecycle contract for E2E test stacks.
+ *
+ * Each concrete adapter (Tauri, Electrobun) implements binary discovery,
+ * WebDriver setup/teardown, and optional PTY daemon management.
+ */
+
+import type { ChildProcess } from 'node:child_process';
+import type { TestFixture } from '../infra/fixtures.ts';
+
+export interface StackCapabilities {
+  /** WebDriver capability object for WDIO */
+  capabilities: Record<string, unknown>;
+}
+
+export abstract class StackAdapter {
+  /** Human-readable stack name (e.g. 'tauri', 'electrobun') */
+  abstract readonly name: string;
+
+  /** WebDriver port for this stack */
+  abstract readonly port: number;
+
+  /** Resolve absolute path to the built binary */
+  abstract getBinaryPath(): string;
+
+  /** Data directory for test isolation */
+  abstract getDataDir(fixture: TestFixture): string;
+
+  /** Build WDIO capabilities object for this stack */
+  abstract getCapabilities(fixture: TestFixture): StackCapabilities;
+
+  /** Spawn the WebDriver process (tauri-driver, WebKitWebDriver, etc.) */
+  abstract setupDriver(): Promise<ChildProcess>;
+
+  /** Kill the WebDriver process */
+  abstract teardownDriver(driver: ChildProcess): void;
+
+  /** Optional: start PTY daemon before tests (Electrobun only) */
+  async startPtyDaemon?(): Promise<ChildProcess>;
+
+  /** Optional: stop PTY daemon after tests */
+  stopPtyDaemon?(daemon: ChildProcess): void;
+}