fix: fail fast on a herdr pane blocked mid-turn, log last-seen agent_status on empty replies

Fixed

• A dialog appearing mid-turn on a herdr pane (after --wait settled on a stale idle/done
  match) polled uselessly until the dispatch's whole deadline expired before reporting an
  unexplained empty reply. Unlike idle/done, a blocked agent_status never resolves on its
  own — it stays blocked until a dialog is answered — so dispatchViaHerdr now fails immediately
  on that status (from both the direct check and from inside waitForHerdrIdle's own polling)
  with the pane's actual dialog text, the same way it already does for a dialog that's there
  before the prompt is even sent.
• A DISPATCH-FAIL outEmpty=true log line gave no way to tell a genuinely empty reply from
  one that simply ran out of deadline while the pane was still busy. The line now includes the
  last-seen agent_status alongside the rendered tail.