feat(docs): Add "Autonomous Trading Agent" with AgentKit Guide

[jadonamite]

Overview

This PR adds a comprehensive guide on building AI Agents for the Base ecosystem using CDP AgentKit and LangChain.

Changes

• New File: apps/base-docs/docs/cookbook/ai-agents/autonomous-trading-agent.md
• Wallet Persistence: Implemented a robust file-based load/save mechanism to solve the common "ephemeral wallet" data loss issue on agent restarts.
• Custom Tooling: Demonstrated how to extend the standard CdpToolkit with a custom Python @tool for specific token balance checks.
• Architecture: Provided a clear template for the ReAct loop (Reason + Act), allowing the agent to chain multiple blockchain interactions from a single prompt.

Why

AI Agents are a rapidly growing vertical. Developers struggle specifically with maintaining agent state (wallet access) across sessions. This guide provides a production-grade pattern for solving that.

[cb-heimdall]

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