fix: notify the triggering herdr pane even when a sprint fails before it starts

Added

• Under HERDR_ENV=1, crew-afk now pushes the sprint's outcome into the triggering pane
  ($HERDR_PANE_ID) once the run finishes, stalls, or errors, instead of leaving that pane to
  poll on a long interval for the same information — see notifyTriggeringPane's doc comment.

Fixed

• That nudge only fired once a sprint had actually been initialized, so a preflight or
  feature-slug resolution failure — the fastest way a run can end — left the triggering pane
  with nothing to stop polling for. main()'s run is now wrapped in a single try/finally
  from before preflight onward, so every exit path, including one that never got as far as
  creating a Sprint, still reaches the notify call.