refactor: standardize runtime guard patterns with lodash-es (#154)

* refactor(ui): replace runtime type guards with lodash-es predicates

* refactor(runtime): migrate typeof checks to lodash-es in core modules

* refactor(ipc): standardize runtime guards with lodash-es

* test(refactor): align runtime guard checks with lodash-es

* docs(agents): add lodash-es runtime guard skill summary

src/utils/account-status.ts

@@ -1,3 +1,5 @@
+import { isString } from 'lodash-es';
+
 export const RATE_LIMIT_HINTS = [
   'resource_exhausted',
   'too many requests',
@@ -40,7 +42,7 @@ export function extractErrorMessage(error: unknown): string {
   if (error instanceof Error) {
     return error.message;
   }
-  if (typeof error === 'string') {
+  if (isString(error)) {
     return error;
   }
   return String(error);