Add security-model discoverability (AGENTS.md -> SECURITY.md -> security model)

Wires the conventional AGENTS.md -> SECURITY.md -> security model chain so automated tooling
can mechanically discover the project's security model. No model content is changed.

patch by Jarek Potiuk; reviewed by Francisco Guerrero, Stefan Miklosovic for CASSANDRA-21464

Assisted-by: Claude Opus 4.8 (1M context)

Author: potiuk

AGENTS.md

@@ -0,0 +1,16 @@
+# 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), which links to the Apache
+Cassandra project security model.
+
+This repository is part of the Apache Cassandra project. Its security model -
+trust boundaries, in-scope / out-of-scope declarations, the security
+properties the project provides and disclaims, and how findings are triaged -
+is the umbrella Cassandra security model linked from SECURITY.md. Consult it
+before reporting security issues.

SECURITY.md

@@ -0,0 +1,17 @@
+# Security Policy
+
+## Reporting a Vulnerability
+
+Please report suspected security vulnerabilities privately to the Apache
+Security Team at <security@apache.org>, following the ASF process at
+<https://www.apache.org/security/>. Do not open public GitHub issues or pull
+requests for security reports.
+
+## Security Model
+
+This repository is part of the Apache Cassandra project. The project's
+security model - what is in and out of scope, the trust boundaries it
+assumes, the security properties it provides and disclaims, and how findings
+are triaged - is documented in the main apache/cassandra repository:
+
+https://github.com/apache/cassandra/blob/trunk/doc/modules/cassandra/pages/reference/security-model.adoc

build.gradle

@@ -179,6 +179,9 @@ rat {
 
   // XML, TXT and HTML reports directory, defaults to 'build/reports/rat'
   reportDir.set(file("build/reports/rat"))
+
+  excludes.add("AGENTS.md")
+  excludes.add("SECURITY.md")
 }
 
 tasks.named('rat').configure {