Phase 8: Define ACL data structure and config schema

[realnatesullivan]

Sub-task of #10.\n\nDesign and implement the ACL data structures and extend node config with allowlist/denylist fields.\n\nAcceptance criteria:\n- [ ] ACL struct (allow/deny lists of peer public keys)\n- [ ] Config schema extended with acl field\n- [ ] Unit tests for ACL parsing and validation