Suggestion: add Base indexer example

### Language Implementation

- [ ] Python
- [x] TypeScript

### Feature Type

- [ ] Action Provider Template
- [ ] Wallet Provider Template
- [ ] Framework Extension
- [ ] Core Requirements
- [x] Other

### 🚀 The feature, motivation and pitch

Hi Coinbase team,
I built a small Node.js Base indexer for learning purposes.
It fetches onchain events from Base and stores them in a local database.
I thought it might be helpful to include a similar example in AgentKit docs for Base developers.

### Alternatives

Another option would be to publish a short tutorial repo showing how to connect AgentKit with Base indexers.

### Additional context

This idea is inspired by Base developer community examples and other public SDKs.