feat: add custom-tool-token-balance tutorial

[jadonamite]

Summary

Adds a tutorial on extending AgentKit with Custom Tools using LangChain and Viem.

Content

• Author: @jadonamite
• Stack: Node.js, Viem, LangChain, Zod.
• Key Features:
  • Implementation of tokenBalanceTool using @langchain/core/tools.
  • Use of viem createPublicClient and readContract for raw blockchain data.
  • Zod schema definition for strict tool inputs.
  • Integration of custom tools into the createReactAgent tools array.

Motivation

Developers often need to extend the default AgentKit capabilities. This guide provides a canonical example of how to write a typed, safe, and effective custom tool for reading contract state.

Verification

• Verified tokenBalanceTool correctly fetches decimals and symbols.
• Verified the tool handles invalid addresses gracefully.
• Verified integration with the existing agent-cli project.

[cb-heimdall]

🟡 Heimdall Review Status

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