refactor: share plain object guard across config and utils

src/infra/plain-object.ts

@@ -0,0 +1,11 @@
+/**
+ * Strict plain-object guard (excludes arrays and host objects).
+ */
+export function isPlainObject(value: unknown): value is Record<string, unknown> {
+  return (
+    typeof value === "object" &&
+    value !== null &&
+    !Array.isArray(value) &&
+    Object.prototype.toString.call(value) === "[object Object]"
+  );
+}