fix(scripts): exempt Storybook stories from coverage:diff gate

Story files are excluded from vitest by design (they run in Storybook
runner, not vitest). Add *.stories.{ts,tsx} to ALLOWED_GLOBS so the
L1 diff gate doesn't flag them as "new untested file".

Also add error-handling test for useOptionalConsent rethrow path
(cookie-consent-banner lines 52-53) achieving 100% statement coverage.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

coverage/summary.json

@@ -1,16 +1,16 @@
 {
-  "generatedAt": "2026-05-19T21:20:25.856Z",
-  "commit": "de17803",
+  "generatedAt": "2026-05-19T21:23:46.645Z",
+  "commit": "1b6f2d1",
   "repo": {
-    "statements": 97.34,
-    "branches": 91.98,
+    "statements": 97.38,
+    "branches": 92.08,
     "functions": 97.09,
-    "lines": 97.34,
+    "lines": 97.38,
     "counts": {
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+      "brf": 1149,
+      "brh": 1058,
       "fnf": 344,
       "fnh": 334
     }
@@ -115,15 +115,15 @@
       }
     },
     "@repo/core-ui": {
-      "statements": 99.49,
-      "branches": 90.2,
+      "statements": 100,
+      "branches": 91.35,
       "functions": 100,
-      "lines": 99.49,
+      "lines": 100,
       "counts": {
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+        "lf": 412,
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+        "brh": 95,
         "fnf": 18,
         "fnh": 18
       }