Function sign_with_api_key 

pub fn sign_with_api_key(
    token: &str,
    wallet_name_or_id: &str,
    chain: &Chain,
    tx_bytes: &[u8],
    index: Option<u32>,
    vault_path: Option<&Path>,
) -> Result<SignResult, OwsLibError>

Sign a transaction using an API token (agent mode).

1. Look up key file by SHA256(token)
2. Check expiry and wallet scope
3. Load and evaluate policies
4. HKDF(token) → decrypt mnemonic
5. HD derive → sign