Function sign_jekt 

pub fn sign_jekt(
    key: &[u8],
    msgid: &str,
    source_agent: &str,
    target_agent: &str,
    ts_secs: i64,
    message: &str,
) -> String

Sign a jekt with the sender’s key. Returns a base64-encoded HMAC-SHA256 tag. key of any length is accepted (HMAC’s own key-hashing handles that) but agent_jekt_keys::agent_jekt_key_ensure always mints 32 bytes.