Opt-in community leaderboard for arena results

[rexblade58]

Context

The portable result schema (schema v1) already records everything needed for fair comparison. A leaderboard should consume only voluntarily submitted results.

Task

1. agenteval arena --submit pushes the portable JSON to a leaderboard endpoint (opt-in, explicit)
2. Leaderboard service (separate repo) validates schema + fairness metadata (commit, environment, agent version)
3. Never upload private code, prompts, logs, or repo names without explicit consent
4. Per-benchmark leaderboards: task pack + repo + agent + model + score

Acceptance criteria

• --submit requires an explicit flag and a LEADERBOARD_URL env var
• Result payload excludes prompts/logs unless --include-artifacts
• Documented submission format matching the portable schema