feat: enable within-file E2E test concurrency for 3x faster runs

Switch all E2E tests from serial test() to testConcurrentIfSelected()
so tests within each file run in parallel. Wall clock drops from ~18min
to ~6min (limited by the longest single test, not sequential sum).

The concurrent helper was already built in e2e-helpers.ts but never
wired up. Each test runs in its own describe block with its own
beforeAll/tmpdir — no shared state conflicts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/skill-e2e-design.test.ts

@@ -560,7 +560,7 @@ describeIfSelected('Design Review E2E', ['design-review-fix'], () => {
     try { fs.rmSync(qaDesignDir, { recursive: true, force: true }); } catch {}
   });
 
-  test('Test 7: /design-review audits and fixes design issues', async () => {
+  testConcurrentIfSelected('design-review-fix', async () => {
     const serverUrl = `http://localhost:${(qaDesignServer as any)?.port}`;
 
     const result = await runSkillTest({