docs: add comprehensive CLAUDE.md for AI assistant guidance

Add detailed documentation for AI assistants working with the codebase:
- Complete codebase structure overview
- Plugin architecture and system documentation
- Development workflows and build processes
- Testing strategy (Jest + Cypress)
- Code style conventions and linting rules
- State management patterns with Immutable.js
- Common tasks and implementation patterns
- Important gotchas and best practices
- Quick reference guide for AI assistants

Author: claude