Previously, AgentError::Aborted was a bare variant that displayed as
just "Aborted", making it impossible to tell whether a session was
killed by wall clock timeout, user cancellation, or something else.
Now it carries an AbortReason (WallClockTimeout or Cancelled) so
terminal output like the retro agent failure message includes the cause.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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