feat: Implement Issue #54 - Add content field instruction capabilities

This implementation adds comprehensive support for content field instructions
that provide guidance for document generation from schemas.

## Key Features Added:

### CLI Options
- `--include-content-instructions` flag to enable content instruction fields
- `--instruction-type` parameter with options: description, example, constraint, template
- Full integration with existing outline mode and heading text capture features

### Schema Generation Enhancements
- Content instruction fields (x-markitect-content-instructions) with contextual guidance text
- Instruction type metadata (x-markitect-instruction-type) for type specification
- Metaschema extension (x-markitect-content-instructions-enabled) for feature detection
- Support for headings, paragraphs, and lists content instructions

### Error Handling
- InvalidInstructionTypeError for robust validation of instruction type parameters
- Comprehensive input validation with clear error messages

### Integration and Compatibility
- Seamless integration with outline mode and heading text capture
- Full backward compatibility - existing behavior unchanged when feature disabled
- Works with all existing CLI options and modes

### Documentation
- Updated CLI help with examples and detailed feature descriptions
- Clear documentation of all instruction types and their purposes

## Technical Implementation:
- Enhanced SchemaGenerator with content instruction generation logic
- Added `_generate_content_instruction` method for contextual instruction text
- Extended schema structure to include instruction metadata
- Maintained clean separation of concerns and existing code patterns

## Testing and Validation:
- Comprehensive test coverage following TDD8 methodology
- All existing functionality preserved and tested
- Integration tests for all feature combinations
- Error handling and edge case validation

This completes Issue #54 with full feature implementation, documentation,
and comprehensive testing coverage.

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

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

markitect/exceptions.py

@@ -168,4 +168,15 @@ class InvalidSchemaError(MarkitectError):
     - Schema doesn't conform to JSON Schema specification
     - Schema file cannot be loaded or parsed
     """
+    pass
+
+
+class InvalidInstructionTypeError(MarkitectError):
+    """Errors related to invalid content instruction types.
+
+    Raised when:
+    - Instruction type is not one of the supported types
+    - Instruction type parameter is malformed
+    - Instruction type conflicts with other options
+    """
     pass