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/stub_generator.py

@@ -34,7 +34,8 @@ class StubGenerator:
 
     def generate_stub_from_schema(self, schema: Dict[str, Any],
                                 placeholder_style: str = 'default',
-                                title: Optional[str] = None) -> str:
+                                title: Optional[str] = None,
+                                schema_file_path: Optional[str] = None) -> str:
         """
         Generate a markdown stub from a JSON schema dictionary.
 
@@ -42,6 +43,7 @@ class StubGenerator:
             schema: JSON schema as dictionary
             placeholder_style: Style of placeholder content ('default', 'custom', 'detailed')
             title: Custom title for the document (overrides schema title)
+            schema_file_path: Optional path to schema file for reference metadata
 
         Returns:
             Generated markdown content as string
@@ -49,9 +51,17 @@ class StubGenerator:
         # Extract title
         doc_title = title or schema.get('title', DEFAULT_TITLE)
 
+        # Check if schema has content instructions enabled
+        content_instructions_enabled = schema.get('x-markitect-content-instructions-enabled', False)
+
         # Start building the markdown content
         lines = []
 
+        # Add schema reference metadata if schema file path is provided
+        if schema_file_path:
+            lines.append(f"<!-- Generated from schema: {schema_file_path} -->")
+            lines.append("")
+
         # Extract heading structure from schema
         headings_schema = schema.get('properties', {}).get('headings', {})
         heading_properties = headings_schema.get('properties', {})
@@ -60,12 +70,12 @@ class StubGenerator:
             # Create a minimal document if no heading structure is defined
             lines.append(f"# {doc_title}")
             lines.append("")
-            lines.append(self._get_placeholder_content(placeholder_style, "main"))
+            lines.append(self._get_placeholder_content(placeholder_style, "main", schema=schema))
             lines.append("")
         else:
             # Generate content based on heading structure
             lines.extend(self._generate_content_from_headings(
-                heading_properties, doc_title, placeholder_style
+                heading_properties, doc_title, placeholder_style, schema=schema
             ))
 
         return '\n'.join(lines)
@@ -90,12 +100,13 @@ class StubGenerator:
         with open(schema_file, 'r', encoding='utf-8') as f:
             schema = json.load(f)
 
-        return self.generate_stub_from_schema(schema)
+        return self.generate_stub_from_schema(schema, schema_file_path=str(schema_file))
 
     def generate_stub_to_file(self, schema: Dict[str, Any],
                             output_file: Path,
                             placeholder_style: str = 'default',
-                            title: Optional[str] = None) -> None:
+                            title: Optional[str] = None,
+                            schema_file_path: Optional[str] = None) -> None:
         """
         Generate a markdown stub and save it to a file.
 
@@ -104,8 +115,9 @@ class StubGenerator:
             output_file: Path where to save the generated markdown
             placeholder_style: Style of placeholder content
             title: Custom title for the document
+            schema_file_path: Optional path to schema file for reference metadata
         """
-        content = self.generate_stub_from_schema(schema, placeholder_style, title)
+        content = self.generate_stub_from_schema(schema, placeholder_style, title, schema_file_path)
 
         # Ensure parent directory exists
         output_file.parent.mkdir(parents=True, exist_ok=True)
@@ -114,7 +126,7 @@ class StubGenerator:
             f.write(content)
 
     def _generate_content_from_headings(self, heading_properties: Dict[str, Any],
-                                      doc_title: str, placeholder_style: str) -> List[str]:
+                                      doc_title: str, placeholder_style: str, schema: Optional[Dict[str, Any]] = None) -> List[str]:
         """Generate markdown content from heading structure."""
         lines = []
 
@@ -129,7 +141,14 @@ class StubGenerator:
             # Start with H1
             lines.append(f"# {doc_title}")
             lines.append("")
-            lines.append(self._get_placeholder_content(placeholder_style, "introduction"))
+            # Get the heading schema for level 1
+            level_1_heading_schema = heading_properties.get('level_1', {})
+            lines.append(self._get_placeholder_content(
+                placeholder_style,
+                "introduction",
+                schema=schema,
+                heading_schema=level_1_heading_schema
+            ))
             lines.append("")
 
             # Generate H2+ headings
@@ -144,7 +163,17 @@ class StubGenerator:
 
                     lines.append(f"{heading_prefix} {section_name}")
                     lines.append("")
-                    lines.append(self._get_placeholder_content(placeholder_style, f"section_level_{level}"))
+
+                    # Get the heading schema for this level
+                    level_key = f"level_{level}"
+                    heading_schema = heading_properties.get(level_key, {})
+
+                    lines.append(self._get_placeholder_content(
+                        placeholder_style,
+                        f"section_level_{level}",
+                        schema=schema,
+                        heading_schema=heading_schema
+                    ))
                     lines.append("")
         else:
             # No H1, start with whatever level is available
@@ -159,7 +188,17 @@ class StubGenerator:
 
                     lines.append(f"{heading_prefix} {section_name}")
                     lines.append("")
-                    lines.append(self._get_placeholder_content(placeholder_style, f"section_level_{level}"))
+
+                    # Get the heading schema for this level
+                    level_key = f"level_{level}"
+                    heading_schema = heading_properties.get(level_key, {})
+
+                    lines.append(self._get_placeholder_content(
+                        placeholder_style,
+                        f"section_level_{level}",
+                        schema=schema,
+                        heading_schema=heading_schema
+                    ))
                     lines.append("")
 
         return lines
@@ -194,8 +233,15 @@ class StubGenerator:
         else:
             return f"Section {index}"
 
-    def _get_placeholder_content(self, style: str, section_type: str) -> str:
+    def _get_placeholder_content(self, style: str, section_type: str, schema: Optional[Dict[str, Any]] = None, heading_schema: Optional[Dict[str, Any]] = None) -> str:
         """Get placeholder content based on style and section type."""
+        # Check if we have content instructions from schema
+        if schema and heading_schema and schema.get('x-markitect-content-instructions-enabled', False):
+            content_instruction = self._extract_content_instruction_from_heading_schema(heading_schema)
+            if content_instruction:
+                return content_instruction
+
+        # Fall back to standard placeholder generation
         generator = self.placeholder_styles.get(style, self.placeholder_styles['default'])
         return generator(section_type)
 
@@ -250,4 +296,26 @@ TODO: Add detailed content for this subsection.""",
         return detailed_placeholders.get(
             section_type,
             f"<!-- {section_type.title()} Section -->\nTODO: Add content for {section_type}."
-        )
+        )
+
+    def _extract_content_instruction_from_heading_schema(self, heading_schema: Dict[str, Any]) -> Optional[str]:
+        """
+        Extract content instruction from a heading schema items definition.
+
+        Args:
+            heading_schema: The schema definition for a heading level
+
+        Returns:
+            Content instruction text if found, None otherwise
+        """
+        # Navigate through the schema structure to find content instructions
+        # Schema structure: heading_schema -> items -> properties -> x-markitect-content-instructions -> const
+        items_schema = heading_schema.get('items', {})
+        if isinstance(items_schema, dict):
+            properties = items_schema.get('properties', {})
+            if isinstance(properties, dict):
+                instruction_schema = properties.get('x-markitect-content-instructions', {})
+                if isinstance(instruction_schema, dict):
+                    return instruction_schema.get('const')
+
+        return None