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/constants.py

@@ -0,0 +1,55 @@
+"""
+Configuration constants and defaults for the markitect assets module.
+
+This module defines default values, file extensions, and other constants
+used throughout the asset management system.
+"""
+
+# Default paths and filenames
+DEFAULT_ASSETS_DIR = "assets"
+DEFAULT_REGISTRY_FILENAME = "asset_registry.json"
+DEFAULT_MANIFEST_FILENAME = "manifest.json"
+
+# Package file extension
+PACKAGE_EXTENSION = ".mdpkg"
+
+# Default configuration values
+DEFAULT_CONFIG = {
+    "enable_deduplication": True,
+    "default_conflict_resolution": "backup",
+    "max_file_size": 100 * 1024 * 1024,  # 100MB
+    "performance_timeout_ms": 100,
+    "memory_limit_mb": 50
+}
+
+# File patterns to exclude from packages by default
+DEFAULT_EXCLUDE_PATTERNS = [
+    ".DS_Store",
+    "Thumbs.db",
+    "*.tmp",
+    "*.temp",
+    "*.swp",
+    "*.bak",
+    "__pycache__",
+    ".git",
+    ".svn",
+    ".hg"
+]
+
+# Supported manifest format version
+MANIFEST_FORMAT_VERSION = "1.0"
+
+# Hash algorithm used for content addressing
+HASH_ALGORITHM = "sha256"
+
+# Symlink conflict resolution options
+CONFLICT_RESOLUTION_OPTIONS = ["overwrite", "backup", "skip"]
+
+# MIME type detection fallbacks
+FALLBACK_MIME_TYPES = {
+    ".md": "text/markdown",
+    ".txt": "text/plain",
+    ".json": "application/json",
+    ".yaml": "application/x-yaml",
+    ".yml": "application/x-yaml"
+}