Add a draft threat model and wire it for discoverability

AGENTS.md

@@ -225,3 +225,9 @@ Fixes #1234, Refs #1234, or None - <short context>
 - Never omit `References`.
 - Use `Refs #...`, `Fixes #...`, or `None - <short context>`.
 - Do not add `Co-authored-by` or AI-assistant trailers to commits or PR descriptions.
+
+## Security
+
+- Read [`SECURITY.md`](SECURITY.md) before reporting or acting on a security issue.
+- Classify security findings against the threat model it links,
+  [`THREAT_MODEL.md`](THREAT_MODEL.md); §11a lists recurring false positives.

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+**Do not report security vulnerabilities through public GitHub issues, pull
+requests, or the mailing lists.**
+
+Report them privately to the Apache Security team:
+
+    security@apache.org
+
+Apache Pekko does not operate a separate project security list; reports go to
+the foundation-wide address above, which routes to the Pekko PMC.
+
+Please follow the [guidelines laid down by the Apache Security
+team](https://www.apache.org/security/). The Pekko PMC will coordinate
+responsible disclosure with affected upstream maintainers where needed.
+
+To receive security announcements, subscribe to the [Apache Announce Mailing
+List](https://lists.apache.org/list.html?announce@apache.org).
+
+## Security Model
+
+Before reporting, please read Apache Pekko's threat model:
+
+[THREAT_MODEL.md](THREAT_MODEL.md)
+
+It states what Pekko treats as a vulnerability and what it does not — in
+particular its assumptions about the network remoting runs on, which
+configuration defaults change the security envelope, and which properties
+Pekko explicitly leaves to the operator. Reports that fall outside the model
+will be closed citing the relevant section, so checking first will save you
+time.
+
+Two points catch most reporters:
+
+- **Pekko remoting assumes a trusted network.** An `ActorSystem` is not
+  designed to be exposed to an untrusted network; it is expected to be
+  protected by network security such as a firewall, and optionally TLS with
+  mutual authentication. See §4 and §7.
+- **All nodes in a Pekko cluster are trusted equally.** There is no
+  Byzantine-peer model — a finding whose precondition is "a cluster member
+  misbehaves" is out of model. See §7.
+
+## Further Security Documentation
+
+- [Apache Pekko security announcements](https://pekko.apache.org/docs/pekko/current/security/)
+- [Remote Security](https://pekko.apache.org/docs/pekko/current/remoting-artery.html#remote-security)
+- [Java Serialization](https://pekko.apache.org/docs/pekko/current/serialization.html#java-serialization)
+- [Remote deployment allow list](https://pekko.apache.org/docs/pekko/current/remoting.html#remote-deployment-allow-list)