refactor(測試): 共用假 Gitea 的啟動與環境變數

每支測試各自寫一次「啟動 stub、登記 close、組環境變數」已經重複三次，抽到
helpers 之後新增測試檔只要一行。各檔仍保留自己的預設路由，因為那是該檔的
情境設定，不該共用。

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/preflight.test.js

@@ -1,17 +1,14 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
 import { runScript, pathWithOnly } from './helpers/run-script.js';
-import { startStubGitea, healthyRoutes } from './helpers/stub-gitea.js';
+import { healthyRoutes, stubEnv as envFor, withStubGitea } from './helpers/stub-gitea.js';
 
 const REPO = 'plugins/tea-sdlc';
 
 async function withStub(t, overrides = {}) {
-  const stub = await startStubGitea(healthyRoutes(REPO, overrides));
-  t.after(() => stub.close());
-  return stub;
+  return withStubGitea(t, healthyRoutes(REPO, overrides));
 }
 
-const envFor = (stub) => ({ TEA_SDLC_API_BASE: stub.base, TEA_SDLC_TOKEN: 'stub-token' });
 
 // ── 第一層：執行環境 ────────────────────────────────────────────────