refactor: share plain object guard across config and utils

src/config/env-preserve.ts

@@ -1,3 +1,5 @@
+import { isPlainObject } from "../infra/plain-object.js";
+
 /**
  * Preserves `${VAR}` environment variable references during config write-back.
  *
@@ -16,15 +18,6 @@
 
 const ENV_VAR_PATTERN = /\$\{[A-Z_][A-Z0-9_]*\}/;
 
-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]"
-  );
-}
-
 /**
  * Check if a string contains any `${VAR}` env var references.
  */