fix: check agent_status directly before giving up on a busy herdr pane that never errors on read

Fixed

• A herdr dispatch could still report an empty reply even after v1.29.79's agent_not_idle
  fix, when the pane was busy in a way that never made agent read itself error. The same
  --wait "does not track turns" gap could leave a read succeeding with code 0 while only
  catching the pane's live status footer (tool-call counters, a running timer) — indistinguishable
  from a genuinely blank reply, and the fixed-delay backoff that follows only covers a few
  seconds of flush lag, not minutes of real work. dispatchViaHerdr now checks agent_status
  directly once that backoff is exhausted: if the pane is still actively busy, it polls until
  idle/done (bounded by the dispatch's own deadline) and reads again, instead of giving up while
  the coder is still working.