feat: live status-board comment — per-model/per-lens review progress

Phase 3 of the gadfly-games build. With several models × several lenses
reviewing a PR, all you'd see mid-run is a row of "⏳ Reviewing…"
placeholders. Add ONE consolidated, live-updating status-board comment
that aggregates every model's per-lens progress (queued → running →
finished + verdict), so progress is visible at a glance and a watcher
can tell when the whole swarm is done.

- cmd/gadfly: opt-in statusWriter (GADFLY_STATUS_FILE) publishes this
  model's lenses to a JSON file, written atomically (temp+rename) as
  runSpecialists transitions each lens. Inert when unset — plain runs
  and tests are unaffected.
- scripts/status-board.sh: background renderer that polls the status
  dir and upserts one marker comment every GADFLY_STATUS_POLL_SECS
  (default 12s), caching the comment id to PATCH in place. Advisory and
  best-effort; the per-model findings comments are untouched.
- entrypoint.sh: pre-seeds every model as queued, launches the board,
  waits only on the review lanes, then signals .done for a final render.
  Default on; disable with GADFLY_STATUS_BOARD=0.
- Docs: README config table + "Live status board" section, example
  stub note, CLAUDE.md architecture map.

gofmt clean, go vet quiet, go build + go test -race green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -28,5 +28,5 @@ RUN apk add --no-cache bash git curl jq ca-certificates
 COPY --from=build /out/gadfly /usr/local/bin/gadfly
 COPY scripts /app/scripts
 COPY entrypoint.sh /entrypoint.sh
-RUN chmod +x /entrypoint.sh /app/scripts/run.sh /usr/local/bin/gadfly
+RUN chmod +x /entrypoint.sh /app/scripts/run.sh /app/scripts/status-board.sh /usr/local/bin/gadfly
 ENTRYPOINT ["/entrypoint.sh"]