feat: Complete Issue #6 - Generate Markdown Stub from Schema

🎯 Core Implementation:
- StubGenerator class with intelligent heading hierarchy generation
- CLI command 'generate-stub' with comprehensive options (--output, --style, --title)
- Multiple placeholder styles: default, custom, detailed
- Full file I/O support and error handling

📊 Features Delivered:
- Template generation from JSON schemas with proper heading structure
- Intelligent section naming based on document hierarchy
- Round-trip validation: generated stubs validate against source schemas
- Integration with existing schema generation and validation workflow

🧪 Quality Assurance:
- 23 comprehensive tests covering all functionality
- Complete TDD8 methodology: RED-GREEN-REFACTOR cycle
- CLI integration tests and error handling validation
- 417/417 total tests passing - no regressions

🔄 Bidirectional Workflow Complete:
Schema Generation (✅ Issue #5) → Schema Validation (✅ Issue #7) → Stub Generation (✅ Issue #6)

This completes the critical template-driven document creation workflow essential
for arc42 architecture documentation system goals.

Usage Examples:
  markitect generate-stub blog_schema.json --output template.md
  markitect generate-stub schema.json --style detailed --title "My Document"

🎖️ Strategic Achievement: Template generation foundation complete and production-ready

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

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

markitect/cli.py

@@ -1419,6 +1419,72 @@ def schema_delete(config, schema_name, confirm):
         sys.exit(1)
 
 
+@cli.command('generate-stub')
+@click.argument('schema_file', type=click.Path(exists=True, path_type=Path))
+@click.option('--output', '-o', type=click.Path(path_type=Path),
+              help='Output file path (default: stdout)')
+@click.option('--style', type=click.Choice(['default', 'custom', 'detailed']),
+              default='default', help='Placeholder content style')
+@click.option('--title', type=str, help='Custom document title')
+@pass_config
+def generate_stub(config, schema_file, output, style, title):
+    """
+    Generate a markdown stub/template from a JSON schema.
+
+    Creates a markdown document with proper heading hierarchy and placeholder
+    content based on the structural definitions in the JSON schema.
+
+    SCHEMA_FILE: Path to the JSON schema file
+
+    Examples:
+        markitect generate-stub blog_schema.json
+        markitect generate-stub schema.json --output template.md
+        markitect generate-stub schema.json --style detailed --title "My Document"
+    """
+    try:
+        if config.get('verbose'):
+            click.echo(f"Generating stub from schema: {schema_file}", err=True)
+
+        from .stub_generator import StubGenerator
+
+        generator = StubGenerator()
+
+        # Load schema and generate stub content
+        import json
+        with open(schema_file, 'r') as f:
+            schema = json.load(f)
+
+        stub_content = generator.generate_stub_from_schema(
+            schema, placeholder_style=style, title=title
+        )
+
+        # Output to file or stdout
+        if output:
+            generator.generate_stub_to_file(schema, output, style, title)
+            click.echo(f"✅ Stub generated: {output}")
+
+            if config.get('verbose'):
+                click.echo(f"Generated markdown template saved to: {output}", err=True)
+        else:
+            click.echo(stub_content)
+
+            if config.get('verbose'):
+                click.echo(f"Generated {len(stub_content)} characters of content", err=True)
+
+    except FileNotFoundError as e:
+        click.echo(f"Error: {e}", err=True)
+        sys.exit(1)
+    except json.JSONDecodeError as e:
+        click.echo(f"Error: Invalid JSON in schema file - {e}", err=True)
+        sys.exit(1)
+    except Exception as e:
+        click.echo(f"Stub generation error: {e}", err=True)
+        if config and config.get('verbose'):
+            import traceback
+            click.echo(traceback.format_exc(), err=True)
+        sys.exit(1)
+
+
 def main():
     """
     Main entry point for the CLI.