fix: clear stale sidecar report.json before each dispatch

Fixed

• runWorker/runReview/runTriage now delete each dispatch's .report.json sidecar
  before writing that round's prompt, instead of leaving a prior round's (or a prior resumed
  sprint's) sidecar sitting at the same fixed path. waitForSidecarReport in dispatch.mjs
  treats existsSync as "found" the instant the herdr wait settles, so a stale sidecar from an
  earlier dispatch to the same slug was read back as this round's verdict whenever the coder's,
  reviewer's, or triage agent's turn died before writing a fresh one — a false verdict rather
  than the intended pane-scrape fallback.