[docs] Add security boundaries reference in SECURITY.md

[evawong-oai]

Summary

1. Add a Security Boundaries section to SECURITY.md.
2. Point readers to the Codex Agent approvals and security documentation for sandboxing, approvals, and network controls.

Validation

1. Reviewed the SECURITY.md diff in a clean worktree.
2. No tests run. Docs only change.