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/tsconfig.json

@@ -7,5 +7,5 @@
     "esModuleInterop": true,
     "skipLibCheck": true
   },
-  "include": ["specs/**/*.ts", "*.ts"]
+  "include": ["specs/**/*.ts", "helpers/**/*.ts", "adapters/**/*.ts", "infra/**/*.ts", "*.ts"]
 }