refactor: dedupe test and script helpers

vitest.pattern-file.ts

@@ -0,0 +1,20 @@
+import fs from "node:fs";
+
+export function loadPatternListFile(filePath: string, label: string): string[] {
+  const parsed = JSON.parse(fs.readFileSync(filePath, "utf8")) as unknown;
+  if (!Array.isArray(parsed)) {
+    throw new TypeError(`${label} must point to a JSON array: ${filePath}`);
+  }
+  return parsed.filter((value): value is string => typeof value === "string" && value.length > 0);
+}
+
+export function loadPatternListFromEnv(
+  envKey: string,
+  env: Record<string, string | undefined> = process.env,
+): string[] | null {
+  const filePath = env[envKey]?.trim();
+  if (!filePath) {
+    return null;
+  }
+  return loadPatternListFile(filePath, envKey);
+}