feat: add leaderboard feature specification

Complete cold-start implementation guide for community leaderboard:
- CLI submit command (agentready submit)
- GitHub Action validation workflow
- Leaderboard aggregation script
- Jekyll pages integrated with existing docs

Enables self-service repository submissions with anti-gaming measures.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jeremyeder