Function agent_send_spawn_env 

pub(crate) async fn agent_send_spawn_env(
    mstore: Arc<Store>,
    id_store: Arc<Store>,
    identity_store: Arc<Store>,
    broker: Option<Arc<Broker>>,
    block: &Block,
    block_id: &str,
    auth_key: &str,
) -> Result<HashMap<String, String>, SpawnGateError>

The environment an agent.send spawn is given: cmd:env through build_persistent_spawn_env, exactly as the agentinput path builds it — identity gate, MuxBus token, wrapper auth, agent identity (UID + token for a row-backed block, AGENTMUX_AGENT_ID from agentName when cmd:env lacks it), git identity, tools PATH. Identity M4b-1 (spec §6.5.8); factored out so the carriage is testable.