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>

markitect/assets/exceptions.py

@@ -0,0 +1,64 @@
+"""
+Asset-specific exception classes for the markitect assets module.
+
+This module provides a hierarchy of exceptions specific to asset management operations,
+following the same patterns as the main markitect exception hierarchy.
+"""
+
+from markitect.exceptions import MarkitectError
+
+
+class AssetError(MarkitectError):
+    """Base exception for all asset management operations.
+
+    Raised when:
+    - Asset file operations fail
+    - Asset validation errors occur
+    - General asset management issues
+    """
+    pass
+
+
+class RegistryError(AssetError):
+    """Errors related to asset registry operations.
+
+    Raised when:
+    - Registry file read/write operations fail
+    - Registry data corruption is detected
+    - Registry validation fails
+    """
+    pass
+
+
+class DeduplicationError(AssetError):
+    """Errors related to asset deduplication operations.
+
+    Raised when:
+    - Deduplication storage operations fail
+    - Symlink creation fails (and fallback fails too)
+    - Asset integrity verification fails
+    """
+    pass
+
+
+class PackagingError(AssetError):
+    """Errors related to package creation and extraction.
+
+    Raised when:
+    - Package creation fails
+    - Package extraction fails
+    - Manifest validation errors
+    - ZIP file operation errors
+    """
+    pass
+
+
+class AssetManagerError(AssetError):
+    """Errors in high-level asset manager operations.
+
+    Raised when:
+    - Configuration validation fails
+    - Component initialization fails
+    - High-level workflow errors occur
+    """
+    pass