[codex] Enforce NIP-OA authorization bounds (BUZZ-SEC-002)

[jmecom]

What changed

The relay now evaluates signed NIP-OA created_at< and created_at> conditions against the signed authentication event on every membership-gated transport. WebSocket auth, HTTP bridge requests, huddles, Git, invites, operator calls, and media reads/uploads all carry the authenticated timestamp into the same verifier.

The ACP harness now treats owner-only literally. It accepts the configured owner pubkey and no longer infers sibling authority from another agent's profile metadata. Explicit allowlists still support intentional multi-actor access.

Why

Buzz previously verified that an owner signed the NIP-OA condition string but did not evaluate its time bounds. Anyone holding an old delegated agent key and credential could therefore reconnect indefinitely while the owner remained a member. Profile-based sibling discovery also let stale or unrelated delegations trigger owner-only agents.

Valid unbounded credentials and credentials whose signed authentication timestamp is inside the requested window continue to work. Direct relay members are unchanged.

Testing

• cargo fmt --all --check
• cargo test -p buzz-sdk nip_oa
• cargo test -p buzz-relay nip_oa_time_conditions
• cargo test -p buzz-acp author_gate_tests