test(skill-validation): exempt deliberate large fixtures from 2MB limit

Pre-existing failure: the "git tracks no files larger than 2MB" test
caught browse/test/fixtures/security-bench-haiku-responses.json (28.8MB
of replay data committed in v1.6.4.0 for security benchmark gate tests).

The test exists to catch accidentally-committed binaries (Mach-O dist
binaries, etc), not to forbid all large files. Add an explicit
LARGE_FIXTURE_EXEMPTIONS allowlist so deliberate replay fixtures pass
the gate while accidental binaries still fail.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/skill-validation.test.ts

@@ -1623,7 +1623,14 @@ describe('no compiled binaries in git', () => {
   test('git tracks no files larger than 2MB', () => {
     // Pure fs.statSync — no shell spawn per file.
     const MAX_BYTES = 2 * 1024 * 1024;
+    // Exempt fixtures that are deliberately tracked at large size (security
+    // benchmark replay data). Add additions to this list with a justification
+    // in the test review trail.
+    const LARGE_FIXTURE_EXEMPTIONS = new Set([
+      'browse/test/fixtures/security-bench-haiku-responses.json',
+    ]);
     const oversized = trackedFiles.filter((f: string) => {
+      if (LARGE_FIXTURE_EXEMPTIONS.has(f)) return false;
       const full = path.join(ROOT, f);
       try {
         return fs.statSync(full).size > MAX_BYTES;