guides/user: Update private repositories

_guides/user.md

@@ -1537,6 +1537,14 @@ left with no real control, forced to blindly trust that their code and data
 won't be viewed, exploited, or even used without consent for purposes like
 training large language models.
 
+> ⚠️
+>
+> While private repositories are supported in Radicle, severe
+> [security vulnerabilities][vuln] make them unfit for use today.
+> The rest of this section remains available for informational purposes.
+
+[vuln]: {% post_url 2026-09-23-disclosure-of-vulnerability-in-network-protocol %}
+
 In contrast, Radicle's decentralized approach puts the power of privacy directly
 in the hands of users by leveraging public key cryptography and granular access
 control mechanisms. The *power to selectively reveal* is a foundational