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.

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

patch by Jarek Potiuk; reviewed by Bret McGuire
reference: https://github.com/apache/cassandra-python-driver/pull/1297