fix: remove Zod transform that breaks Reed AI tools

[igor47]

Summary

• Remove .transform((v) => v ?? 0) from CoinDelta schema in updateCoins.ts
• Zod v4's toJSONSchema cannot serialize .transform() calls, causing Transforms cannot be represented in JSON Schema errors when the AI SDK tries to convert tool schemas
• The null-to-0 coercion is already handled downstream with || 0

Test plan

• Verify Reed responds to chat messages in production after deploy
• Verify coin update tool works correctly (gain/spend coins via Reed)

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