Add AGENTS.md with security-model link for agent discoverability

Adds an AGENTS.md entry point so automated scan agents can discover the
project's security model via the conventional AGENTS.md -> SECURITY.md
chain. No existing content is changed.

Generated-by: Claude Code (Claude Opus 4.8)

Author: potiuk

AGENTS.md

@@ -0,0 +1,13 @@
+# Agent guidance
+
+This file is read by automated agents (security scanners, code analyzers,
+AI assistants) operating on this repository. It points them at the
+human-authored references they should consult before producing output.
+
+## Security
+
+Security model: [SECURITY.md](./SECURITY.md)
+
+Agents that scan this repository should consult `SECURITY.md` for the
+project's threat model, in-scope / out-of-scope declarations, and known
+non-findings before reporting issues.