feat: complete Issue #149 - Phase 2: Implement Explode-Implode Variants

Implement all three explode-implode variants with full CLI integration:

🔧 Variant Implementations:
- FlatVariant: Encapsulates existing flat structure behavior
- HierarchicalVariant: Numbered directory structures (01_, 02_, 03_)
- SemanticVariant: Content-based organization (intro, chapters, appendices)

🏭 Factory System:
- VariantFactory: Centralized variant creation and management
- Auto-detection algorithms with confidence scoring
- Content analysis for variant recommendation

🖥️ CLI Integration:
- Enhanced md-explode command with --variant parameter
- Enhanced md-implode command with auto-detection
- Improved error handling and user feedback

🧪 Comprehensive Testing:
- 22 unit tests covering all variant functionality
- Roundtrip validation ensuring perfect reversibility
- Performance testing with large documents
- Error handling and edge case coverage

📊 Key Features:
- Three distinct organization strategies
- Automatic variant detection from directory structures
- Full backward compatibility with existing behavior
- Extensible architecture for future variants
- Manifest-based reversibility

Files Added:
- markitect/explode_variants/flat_variant.py
- markitect/explode_variants/hierarchical_variant.py
- markitect/explode_variants/semantic_variant.py
- markitect/explode_variants/variant_factory.py
- tests/test_issue_149_explode_implode_variants.py
- tests/test_issue_149_roundtrip_validation.py
- cost_notes/issue_149_cost_2025-10-12.md

Files Modified:
- markitect/explode_variants/__init__.py (updated exports)
- markitect/plugins/builtin/markdown_commands.py (CLI integration)

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

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

markitect/explode_variants/__init__.py

@@ -15,6 +15,10 @@ from .enums import ExplodeVariant, ExplodeMode, ManifestVersion, DetectionConfid
 from .base_variant import BaseVariant, ExplodeOptions, ImplodeOptions, ExplodeResult, ImplodeResult
 from .manifest_manager import ManifestManager, ManifestData, StructureEntry
 from .variant_detector import VariantDetector, DetectionResult
+from .flat_variant import FlatVariant
+from .hierarchical_variant import HierarchicalVariant
+from .semantic_variant import SemanticVariant
+from .variant_factory import VariantFactory, get_variant_factory, create_variant, detect_variant, auto_create_variant
 
 __all__ = [
     'ExplodeVariant',
@@ -30,5 +34,13 @@ __all__ = [
     'ManifestData',
     'StructureEntry',
     'VariantDetector',
-    'DetectionResult'
+    'DetectionResult',
+    'FlatVariant',
+    'HierarchicalVariant',
+    'SemanticVariant',
+    'VariantFactory',
+    'get_variant_factory',
+    'create_variant',
+    'detect_variant',
+    'auto_create_variant'
 ]