Remove: Rabbit Treasury, Burrow math, vault sinks, treasury UX

Context

Remove Rabbit Treasury / Burrow from Arena v2. Parent epic: #238 (closed).

Relevant files

• contracts/src/RabbitTreasury.sol, contracts/src/libraries/BurrowMath.sol
• contracts/src/sinks/RabbitTreasuryVault.sol
• docs/product/rabbit-treasury.md, docs/onchain/treasury-contracts.md
• skills/play-rabbit-treasury/SKILL.md
• frontend/src/pages/RabbitTreasuryPage.tsx
• Indexer: idx_rabbit_* tables, GET /v1/rabbit/*

Acceptance criteria

• No FeeRouter 10% Rabbit sink in v2 deploy
• Doubloon.mint minter role repointed (TimeArena or governance only)
• All Burrow indexer endpoints removed

Verification checklist

• rg -i "rabbit.?treasury|burrow" clean in contracts, indexer, frontend, docs, skills
• DeployDev succeeds without RabbitTreasury