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-qa-workflow.test.ts

@@ -37,7 +37,7 @@ describeIfSelected('QA skill E2E', ['qa-quick'], () => {
     try { fs.rmSync(qaDir, { recursive: true, force: true }); } catch {}
   });
 
-  test('/qa quick completes without browse errors', async () => {
+  testConcurrentIfSelected('qa-quick', async () => {
     const result = await runSkillTest({
       prompt: `B="${browseBin}"
 
@@ -108,7 +108,7 @@ describeIfSelected('QA-Only skill E2E', ['qa-only-no-fix'], () => {
     try { fs.rmSync(qaOnlyDir, { recursive: true, force: true }); } catch {}
   });
 
-  test('/qa-only produces report without using Edit tool', async () => {
+  testConcurrentIfSelected('qa-only-no-fix', async () => {
     const result = await runSkillTest({
       prompt: `IMPORTANT: The browse binary is already assigned below as B. Do NOT search for it or run the SKILL.md setup block — just use $B directly.
 
@@ -227,7 +227,7 @@ describeIfSelected('QA Fix Loop E2E', ['qa-fix-loop'], () => {
     try { fs.rmSync(qaFixDir, { recursive: true, force: true }); } catch {}
   });
 
-  test('/qa fix loop finds bugs and commits fixes', async () => {
+  testConcurrentIfSelected('qa-fix-loop', async () => {
     const qaFixUrl = `http://127.0.0.1:${qaFixServer!.port}`;
 
     const result = await runSkillTest({