docs: add CLAUDE.md for Claude Code guidance

[AlexCheema]

Motivation

Claude Code (claude.ai/code) uses CLAUDE.md files to understand project context. This file helps future Claude Code sessions be more productive by providing build commands, architecture overview, and code style requirements specific to this repo.

Changes

Added CLAUDE.md with:

• Build and run commands (dashboard build, uv run exo, pytest, basedpyright, ruff)
• Architecture overview (Node composition, message flow, event sourcing pattern)
• Key type hierarchy locations
• Code style requirements from .cursorrules

Why It Works

CLAUDE.md is automatically read by Claude Code when starting a session. Having project-specific guidance reduces onboarding time and ensures Claude follows the project's conventions.

Test Plan

Manual Testing

N/A - documentation only

Automated Testing

N/A - documentation only