Reject malformed closing fences

Authored-By: Codex (GPT-5) <noreply@openai.com>

check.sh

@@ -29,6 +29,10 @@ if printf '%s\n' "${BODY}" | grep -Fq '<!--' ||
   echo "FAIL[check-naming]: PR body must not contain HTML comment delimiters"
   FAILED=1
 fi
+if printf '%s\n' "${BODY}" | grep -Eq '^[[:blank:]]*(```|~~~)'; then
+  echo "FAIL[check-naming]: PR body must not contain fenced code delimiters"
+  FAILED=1
+fi
 
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