feat: add JUDGING.md spec for pairwise game jam judging (#2)

Comprehensive specification for pairwise-preference judging system:
- Discord OAuth with Sensei role verification
- GitHub as source of truth for entries (entries.yaml)
- Active ranking with Beta distribution uncertainty sampling
- Spectral (PageRank-style) ranking algorithm
- Minimal schema: single jam_comparisons table
- 6 API endpoints total

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: kronosapiens