Function sign_channel_jekt 

pub fn sign_channel_jekt(
    private_key: &[u8],
    msgid: &str,
    source_agent: &str,
    source_channel: &str,
    target_agent: &str,
    ts_secs: i64,
    message: &str,
) -> Option<String>

Sign a cross-channel jekt with the sending agent’s own LAN private key (AGENTMUX_LAN_KEY) — called client-side in agentmux-mcp, never server-side, exactly like sign_lan_jekt.

private_key must be exactly 32 bytes (the seed generate_lan_keypair produced); returns None on any other length rather than panicking — same “a missing or malformed key must never block sending” policy the other signers use. A message that can’t be signed is delivered unsigned, and the receiver decides what that means.