Docs: Create Codebase Overview Guide

As part of the agent protocol simplification epic (#7604), we need a new guide to provide a high-level conceptual map of the Neo.mjs codebase. This guide will replace the need for the agent to parse the large class-hierarchy.yaml file for initial discovery.

Acceptance Criteria:

1. Create a new file at learn/guides/fundamentals/CodebaseOverview.md.
2. The guide should explain the "batteries included" philosophy of the framework.
3. It must provide a high-level overview of the major namespaces within the src directory (e.g., component, data, form, grid, manager, worker, etc.), explaining their purpose and how they relate to each other.
4. Add the new guide to learn/tree.json under the "Fundamentals & Core Concepts" section.