Update documentation to highlight top-level Agent import

[codegen-sh]

This PR updates the documentation to highlight that the Agent class can be imported directly from the top-level package using from codegen import Agent.

The Agent class was already properly exposed in the top-level __init__.py file, but the documentation examples were still using the longer import path. This PR updates:

1. The main README.md to show the Agent import in the usage examples
2. The agents/README.md to show both import options with the top-level import as the recommended approach

This addresses the request from @jay Hack to make the Agent class available as a top-level import.

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