feat: Implement comprehensive lexicon validation system

- Add internal/validation/lexicon.go with ValidateLexiconData function
- Create validate-lexicon CLI tool for testing lexicon schemas
- Add comprehensive test suite with valid and invalid test cases
- Include test data for actors, communities, posts, interactions, and moderation
- Replace shell-based validation script with Go implementation
- Support for complex validation including enums, unions, and references

This provides a robust foundation for validating atProto lexicon data
in the Coves platform, ensuring data integrity and type safety.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

feat: Add domain knowledge documentation and update lexicon schemas