chore: add CLAUDE.md file

[phatblat]

This PR adds a CLAUDE.md file to save some tokens on future Claude Code sessions.

While generating this, Claude found deviations in the data model implementation that should be revisited at some point. Logged in SDKS-1434

⏺ You're right to point that out. Looking at the actual data model implementations across the codebase, there are indeed variations in field
naming between platforms:

1. Android/Kotlin uses: title, done, deleted
2. JavaScript/React/Swift/Rust use: text, isCompleted
3. Some implementations include a deleted field for soft deletes, others don't

This inconsistency across platforms could cause interoperability issues since these apps are supposed to sync data between devices. The
CLAUDE.md documentation correctly notes these variations, which is important for developers to understand when working across different
platform implementations.

[Copilot]

Pull Request Overview

This PR adds a CLAUDE.md file to provide context and guidance for Claude Code when working with this multi-platform Ditto SDK quickstart repository. The file serves as documentation to help reduce token usage in future Claude sessions by providing pre-loaded context about the repository structure, development commands, and architecture patterns.

Key changes:

• Comprehensive documentation of the repository's multi-platform structure and purpose
• Development command references for all supported platforms (Android, JavaScript, Rust, Flutter, .NET, C++)
• Architecture overview including Ditto SDK integration patterns and platform-specific implementations
• Notes about data model inconsistencies across platforms that need future attention