Remove: Leprechaun NFT contracts, pages, docs, skills

Context

Remove Leprechaun NFT surface from Arena v2 full redeploy. Parent epic: #238 (closed).

Relevant files

• contracts/src/LeprechaunNFT.sol
• docs/product/leprechaun-nfts.md
• skills/collect-leprechaun-sets/SKILL.md
• frontend/src/pages/CollectionPage.tsx (if present)
• contracts/script/DeployProduction.s.sol, DeployDev.s.sol
• Indexer: leprechaun event tables/migrations (grep leprechaun)

Acceptance criteria

• Contract, tests, deploy wiring removed
• Frontend route/nav removed
• Docs + skills deleted or archived with "removed in Arena v2" stub
• Guardrails skill (.cursor/skills/yieldomega-guardrails/SKILL.md) leprechaun bullets removed

Verification checklist

• rg -i leprechaun returns zero hits outside CHANGELOG/archive
• forge build + indexer build green