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/cli.py

@@ -5678,11 +5678,123 @@ markitect graphql-serve --port 5000
 # Query running server
 markitect graphql-query "{ markdownFiles { filename } }" \\
   --endpoint http://localhost:5000/graphql
+
+🛠️ Mutation Examples (Write Operations):
+
+# Add a new markdown file
+markitect graphql-mutate \\
+  'mutation { addMarkdownFile(filename: "new-doc.md", content: "# New Document\\n\\nContent here") { success markdownFile { id filename } errors } }' \\
+  --local
+
+# Update existing file
+markitect graphql-mutate \\
+  'mutation { updateMarkdownFile(id: 1, content: "# Updated\\n\\nNew content") { success errors } }' \\
+  --local
+
+# Add a JSON schema
+markitect graphql-mutate \\
+  'mutation { addSchema(filename: "user-schema.json", schemaContent: "{\\"type\\": \\"object\\", \\"properties\\": {\\"name\\": {\\"type\\": \\"string\\"}}}") { success schema { id title } errors } }' \\
+  --local
+
+# Delete a schema
+markitect graphql-mutate \\
+  'mutation { deleteSchema(filename: "old-schema.json") { success deletedFilename errors } }' \\
+  --local
+
+💡 Access GraphQL Playground at http://localhost:5000/graphql when server is running
 """
 
     click.echo(examples)
 
 
+@cli.command(name='graphql-mutate')
+@click.argument('mutation', required=True)
+@click.option('--variables', default='{}', help='JSON string of mutation variables')
+@click.option('--endpoint', default='http://localhost:5000/graphql', help='GraphQL endpoint URL')
+@click.option('--local', is_flag=True, help='Execute mutation locally without HTTP')
+@click.option('--format', 'output_format', type=click.Choice(['json', 'yaml', 'table']),
+              default='json', help='Output format')
+@pass_config
+def graphql_mutate(config, mutation, variables, endpoint, local, output_format):
+    """Execute GraphQL mutations for write operations.
+
+    Execute GraphQL mutations to add, update, or delete data in MarkiTect.
+    Supports both local and remote execution modes.
+
+    Examples:
+        # Add a new markdown file locally
+        markitect graphql-mutate 'mutation { addMarkdownFile(filename: "test.md", content: "# Test\\nContent") { success markdown_file { id filename } } }' --local
+
+        # Update an existing file
+        markitect graphql-mutate 'mutation { updateMarkdownFile(id: 1, content: "# Updated\\nContent") { success errors } }' --local
+
+        # Add a schema with variables
+        markitect graphql-mutate 'mutation($filename: String!, $content: JSONString!) { addSchema(filename: $filename, schemaContent: $content) { success schema { id title } } }' --variables '{"filename": "test.json", "content": "{\\"type\\": \\"object\\"}"}' --local
+    """
+    import sys
+    import json
+    import yaml
+
+    try:
+        # Parse variables
+        try:
+            mutation_variables = json.loads(variables) if variables != '{}' else {}
+        except json.JSONDecodeError:
+            click.echo("❌ Invalid JSON in variables parameter", err=True)
+            sys.exit(1)
+
+        if local:
+            # Execute locally using the GraphQL client
+            try:
+                from .graphql import GraphQLClient
+                client = GraphQLClient()
+                result = client.execute_local(mutation, variables=mutation_variables)
+            except ImportError as e:
+                click.echo(f"❌ Missing dependency: {e}", err=True)
+                sys.exit(1)
+        else:
+            # Execute remotely
+            try:
+                from .graphql import GraphQLClient
+                client = GraphQLClient(endpoint)
+                result = client.execute(mutation, variables=mutation_variables)
+            except ImportError as e:
+                click.echo(f"❌ Missing dependency: {e}", err=True)
+                sys.exit(1)
+
+        # Format and display output
+        if output_format == 'json':
+            click.echo(json.dumps(result, indent=2))
+        elif output_format == 'yaml':
+            click.echo(yaml.dump(result, default_flow_style=False))
+        elif output_format == 'table':
+            # For mutations, simple table output
+            if result.get('data'):
+                click.echo("Mutation Result:")
+                for key, value in result['data'].items():
+                    if isinstance(value, dict):
+                        click.echo(f"  {key}:")
+                        for sub_key, sub_value in value.items():
+                            click.echo(f"    {sub_key}: {sub_value}")
+                    else:
+                        click.echo(f"  {key}: {value}")
+
+            if result.get('errors'):
+                click.echo("Errors:")
+                for error in result['errors']:
+                    click.echo(f"  - {error.get('message', error)}")
+
+    except ImportError as e:
+        click.echo(f"❌ Missing dependency: {e}", err=True)
+        sys.exit(1)
+    except Exception as e:
+        click.echo(f"❌ Failed to execute mutation: {e}", err=True)
+        if config.get('verbose'):
+            import traceback
+            click.echo(traceback.format_exc(), err=True)
+        sys.exit(1)
+
+
 # Register issue management commands
 cli.add_command(issues_group)