feat: Implement Issue #55 - Schema-based draft generation with content instructions

This implementation enhances the existing generate-stub command to utilize
content field instructions from schemas, providing guided document generation
with specific placeholder text instead of generic "TODO" messages.

## Key Features Added:

### Enhanced Schema-Based Generation
- Content instructions from schemas (x-markitect-content-instructions) are now used
- Schema reference metadata included in generated drafts for traceability
- Intelligent fallback to generic placeholders for schemas without instructions
- Full integration with existing generate-stub CLI command and options

### StubGenerator Enhancements
- New _extract_content_instruction_from_heading_schema method for instruction parsing
- Enhanced _get_placeholder_content method with schema-aware content generation
- Updated method signatures to support schema_file_path parameter throughout
- Robust handling of both content instruction and legacy schema formats

### CLI Integration
- Updated generate-stub command documentation with content instruction examples
- Enhanced help text explaining automatic content instruction usage
- Fixed output file generation to include schema references correctly
- Maintained full backward compatibility with existing usage patterns

### Technical Implementation
- Schema reference comments (<!-- Generated from schema: path -->) in generated drafts
- Content instruction text extracted from x-markitect-content-instructions fields
- Support for all instruction types (description, example, constraint, template)
- Integration with existing heading hierarchy and placeholder style systems

## Integration and Compatibility:
- Seamless integration with Issue #54 content field instructions
- Full backward compatibility with existing schemas and usage
- Works with outline mode schemas and heading text capture features
- Comprehensive error handling and graceful degradation

## Testing and Validation:
- Comprehensive test suite covering all acceptance criteria
- Integration tests with schema-generate → generate-stub workflow
- Validation of schema reference metadata and content instruction usage
- Backward compatibility testing with legacy schemas

This completes Issue #55 with full feature implementation, comprehensive testing,
and enhanced documentation for schema-based draft generation capabilities.

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

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

markitect/cli.py

@@ -1984,7 +1984,9 @@ 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.
+    content based on the structural definitions in the JSON schema. When schemas
+    include content instructions (x-markitect-content-instructions), the generated
+    stub will use specific guidance text instead of generic placeholders.
 
     SCHEMA_FILE: Path to the JSON schema file
 
@@ -1992,6 +1994,19 @@ def generate_stub(config, schema_file, output, style, title):
         markitect generate-stub blog_schema.json
         markitect generate-stub schema.json --output template.md
         markitect generate-stub schema.json --style detailed --title "My Document"
+
+        # Content instructions will be used automatically when present in schema
+        markitect generate-stub schema_with_instructions.json
+
+    Content Instructions:
+        When a schema contains x-markitect-content-instructions-enabled: true,
+        the generated stub will include specific content guidance from the schema
+        instead of generic "TODO" placeholders. This is especially useful with
+        schemas created using the --include-content-instructions option.
+
+    Schema Reference:
+        Generated stubs include a comment referencing the source schema file
+        for validation and traceability purposes.
     """
     try:
         if config.get('verbose'):
@@ -2009,7 +2024,7 @@ def generate_stub(config, schema_file, output, style, title):
             schema = json.load(f)
 
         stub_content = generator.generate_stub_from_schema(
-            schema, placeholder_style=style, title=title
+            schema, placeholder_style=style, title=title, schema_file_path=schema_file
         )
 
         # Mode-based output logic
@@ -2021,7 +2036,7 @@ def generate_stub(config, schema_file, output, style, title):
 
         # Output to file or stdout
         if output:
-            generator.generate_stub_to_file(schema, output, style, title)
+            generator.generate_stub_to_file(schema, output, style, title, schema_file)
             click.echo(f"✅ Stub generated: {output}")
 
             if config.get('verbose'):