Function spawn_long_running 

pub fn spawn_long_running(
    builtin: &'static str,
    program: String,
    args: Vec<String>,
    cwd: Option<PathBuf>,
    env: BTreeMap<String, String>,
    session_id: String,
) -> Result<LongRunningHandleInfo, HostlibError>

Spawn the argv as a long-running child process and return a handle.

The background waiter pushes a tool_result entry into the active session’s agent_inbox when the process exits so the next agent-loop turn sees the result.