refactor: clean up repo root, remove duplicate sandbox dirs

- Delete root-level warpgrep-vercel-sandbox/ and warpgrep-cloudflare-sandbox/
  (duplicates of warpgrep/vercel-sandbox/ and warpgrep/cloudflare-sandbox/)
- Move Dockerfile and fly.toml into github_app/ where they belong
- Move eval_results.md into pr_review_agent/
- Move online_eval.py into pr_review_agent/ (update ETL path)
- Update README links to point to warpgrep/ subdirs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>