Function explain_failure 

async fn explain_failure(
    exit_code: Option<i32>,
    signal: Option<i32>,
    result_frame: Option<Value>,
    stderr_rx: Receiver<Vec<u8>>,
) -> String

Await the captured stderr, classify the exit, log a warning, and render the human-readable explanation that becomes the run’s terminal error string. See docs/specs/SPEC_AGENT_FAILURE_DIAGNOSTICS_2026_06_11.md.