/// The outcome of an Agent turn or run.
///
/// Represents the state after a turn completes:
/// - `Completed` — task finished successfully; run ends.
/// - `Continuing` — turn ended but the run is still in progress (guard nudge).
/// - `Failed` — unrecoverable error; run ends.
/// - `MaxTurnsExceeded` — hit the turn cap; run ends.
/// - `Cancelled` — user or system cancelled; run ends.