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/gemini-e2e.test.ts

@@ -76,7 +76,7 @@ if (evalsEnabled && !process.env.EVALS_ALL) {
 /** Skip an individual test if not selected by diff-based selection. */
 function testIfSelected(testName: string, fn: () => Promise<void>, timeout: number) {
   const shouldRun = selectedTests === null || selectedTests.includes(testName);
-  (shouldRun ? test : test.skip)(testName, fn, timeout);
+  (shouldRun ? test.concurrent : test.skip)(testName, fn, timeout);
 }
 
 // --- Eval result collector ---