Added comprehensive GraphQL mutations for CRUD operations on markdown files and schemas.
Key features:
- Complete mutation schema with structured payload types
- Markdown file mutations: add, update with front matter support
- Schema mutations: add, update, delete with JSON validation
- CLI integration with `graphql-mutate` command
- Comprehensive error handling and validation
- Full test coverage with 24 test cases
- Updated documentation with mutation examples and API usage
Resolves issue #10: Expose a GraphQL Write Interface
🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>
