feat: implement GraphQL write interface with mutations (issue #10)

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>

markitect/graphql/__init__.py

@@ -7,6 +7,6 @@ database content including Markdown files, ASTs, and schemas.
 
 from .schema import schema
 from .server import GraphQLServer, GraphQLClient
-from .resolvers import Query
+from .resolvers import Query, Mutation
 
-__all__ = ['schema', 'GraphQLServer', 'GraphQLClient', 'Query']
+__all__ = ['schema', 'GraphQLServer', 'GraphQLClient', 'Query', 'Mutation']