docs: link to agent-skills repo for custom verifier skill

Add a link to the build-custom-user-verifier agent skill hosted in
ArcadeAI/agent-skills, giving coding agents an LLM-friendly reference
for implementing the verification flow.

Made-with: Cursor

Author: byrro

app/en/guides/user-facing-agents/secure-auth-production/page.mdx

@@ -53,6 +53,11 @@ If you need help, join the [Implementing a custom user verifier](https://github.
   Python Flask app you can run locally to test the verification flow, use as a
   reference when building your own, or to provide as a sample to your coding
   agent (e.g. Claude Code, Cursor Agent, etc).
+
+  If you're using a coding agent to help build your verifier, give it the
+  [build a custom user verifier](https://github.com/ArcadeAI/agent-skills/tree/main/build-custom-user-verifier)
+  skill — it contains the full flow, pseudocode, and common mistakes in an
+  LLM-friendly format.
 </Callout>
 
 import { Steps, Tabs, Callout } from "nextra/components";