restore: SECURITY_STUB.md — accidentally removed from public repo

Author: Neo

SECURITY_STUB.md

@@ -0,0 +1,15 @@
+# Security Layer
+
+0pnMatrx includes a closed-source security layer that is not part of this repository.
+
+The security layer handles:
+- Agent boundary enforcement
+- Constraint manifest validation
+- Audit trail and integrity verification
+- The Neo access protection protocol
+- Ban system and blockchain attestation
+- Owner-level access verification
+
+This layer is closed source by design. It cannot be bypassed, forked, or replicated from this repository. It connects to the open source runtime through the interface documented in `runtime/security/SECURITY_INTERFACE.md`.
+
+If you are building on 0pnMatrx, you do not need the security layer to run the platform locally. It is required only for production deployments serving public users.