Function classify 

pub fn classify(
    exit_code: Option<i32>,
    signal: Option<i32>,
    stderr: &str,
    result_frame: Option<&Value>,
) -> AgentFailure

Classify an agent run failure.

exit_code / signal come from the child’s ExitStatus. stderr is the captured (capped) child stderr. result_frame is the terminal stream-json result object when one was seen — the CLI sometimes reports an error there while still exiting 0, so it is folded into the evidence.