Function export_to 

pub fn export_to(
    path: &Path,
    engine: &GovernanceEngine,
    limits: &Limits,
    signing_key: &SigningKey,
) -> Result<ParentPolicy, ParentPolicyError>

User side: export and atomically write path (normally <run>/<id>/parent-policy.json). The version continues from whatever valid policy the file already holds.