Add scoped merchant credit MVP

[max-digi]

Summary

• add scoped merchant credit storage and event tracking
• add /tip credit @account 2 prospectbutcher with confirm and spend actions
• stub Tempo TIP-1015 issuance and MPP receipt paths for the MVP

Tests

• pnpm check
• pnpm test src/lib/scopedCredit.test.ts
• pnpm test src/chat.workers.test.ts -t "/tip credit creates and spends merchant-scoped credit"