Add a Base network example and integration tutorial

### Language Implementation

- [ ] Python
- [ ] TypeScript

### Feature Type

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

### 🚀 The feature, motivation and pitch

Although AgentKit supports multiple networks (including Base, Ethereum, and Solana), there is limited step-by-step example/tutorial content specifically focused on building and deploying agents on Base. A Base-focused example would help developers understand AgentKit usage in a real-world Base ecosystem context.

Expected behavior:
- An example project demonstrating a full Base agent workflow: wallet creation, connecting to Base testnet, handling payments, and responding to onchain events.
- Include clear setup instructions, environment variable requirements (e.g., CDP API key), and code snippets illustrating typical tasks.
- Tutorial should be included under repository `examples/` folder and referenced in the main README.

### Alternatives

Proposed solution:
- Create a directory `examples/base-agent` with code samples (Node.js and Python, if applicable).
- Document how to run the example locally and connect it to the Base network.
- Add a section in the README to highlight the Base example and link to deeper “Building on Base” docs.

Acceptance criteria:
- Base example runs end-to-end with minimal setup and is documented in the main README.
- Tutorial includes “Expected outputs” and screenshots/logs showing successful deployments.
- Contributors can follow the example without external guides.

### Additional context

_No response_