cleanup: remove pm-data-freshness bench and all dead refs - #1666

Summary

• Remove all references to pm-data-freshness bench (deleted in prior PR) and dead Codex data feed
• Drop 3 dead Prometheus queries (pm_freshness_delta_ms_bucket, pm_health) from pm-stats.ts
• Remove codex from PM_DATA_FEEDS (feed is dead)
• Add pm-ws-latency to bench-published.json, remove pm-data-freshness
• Update FAQ/methodology copy in pm-api-latency.yml and pm-rate-limits.yml to point at pm-ws-latency instead
• Minor comment cleanups in format.ts, pm-data-feeds-leaderboard.tsx, benchmarks/[slug]/page.tsx

Test plan

• /prediction-markets hub renders without errors (codex row gone from data feeds tab)
• /benchmarks/pm-api-latency and /benchmarks/pm-rate-limits FAQ copy no longer references deleted bench
• No stale pm-data-freshness slug anywhere in src/ or benchmarks/