test: lower the claude design-review size pin

The size guard (#3199) asks for it: claude/design-review shrank by 437 chars
without its "When to invoke" section.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/fixtures/skill-md-size-allowlist.json

@@ -10,7 +10,7 @@
     "agy/gstack-plan-devex-review": 135727,
     "agy/gstack-plan-eng-review": 152669,
     "agy/gstack-review": 124266,
-    "claude/design-review": 136826,
+    "claude/design-review": 136389,
     "codex/gstack-autoplan": 130298,
     "codex/gstack-design-consultation": 109239,
     "codex/gstack-design-review": 144820,