fix: report render line count and echo-found status on herdr empty-reply failures

Changed

• A herdr DISPATCH-FAIL outEmpty=true line now reports the captured render's line count and
  whether the echoed prompt was found anywhere in it (lines=N echoFound=yes|no). Previously
  the log's only diagnostic was the last 400 characters of the render — which is always just
  Claude's bottom status-bar chrome whether the reply capture failed or succeeded, so it couldn't
  distinguish "the read's --lines window (or the pane's alt-screen buffer) never captured the
  echoed prompt at all" from "the echo is there but extractHerdrReply's end-marker pattern
  didn't match." echoFound is computed by herdrEchoFound, sharing herdrIsEchoLine with
  extractHerdrReply itself rather than reimplementing the echo check.