feat: comprehensive asset management system and testing improvements

Asset Management System (Issue #142):
- Add complete asset management framework with deduplication
- Implement AssetManager, AssetRegistry, and AssetDeduplicator classes
- Add AssetPackager for markdown document packaging
- Create comprehensive test suite for all asset management components
- Add asset constants and custom exceptions for robust error handling

Markdown Processing Enhancements:
- Update markdown_commands.py with improved functionality
- Enhanced parsing and content aggregation capabilities
- Improved filename encoding/decoding for special characters

Test Suite Improvements:
- Add comprehensive tests for Issue #138 markdown parsing
- Enhance Issue #139 content aggregation and end-to-end testing
- Complete test coverage for new asset management features

Examples and Documentation:
- Update BildungsKanonJon.md example with enhanced content
- Generate corresponding HTML output for documentation
- Add asset registry configuration

Development Tools:
- Add install script for simplified setup

This commit represents a major enhancement to MarkiTect's asset handling
capabilities with full test coverage and improved markdown processing.

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

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

tests/test_issue_139_content_aggregation.py

@@ -365,7 +365,7 @@ More content""")
             file_path.write_text(content)
             files.append(file_path)
 
-        aggregated = aggregate_content(files, preserve_front_matter=True)
+        aggregated = aggregate_content(self.temp_dir, preserve_front_matter=True)
 
         # Should have front matter at the beginning
         lines = aggregated.split('\n')