feat: complete TDD8 implementation of markdown file explosion - Issue #138

Complete implementation of md-explode command for transforming single
markdown files into organized directory structures:

Core Implementation:
- MarkdownSection class for hierarchical document modeling
- extract_headings() - Parse markdown headings with levels
- parse_markdown_structure() - Build section hierarchy from content
- generate_safe_filename() - Convert headings to filesystem-safe names
- explode_markdown_file() - Main explosion functionality
- DirectoryStructureBuilder - Create organized file/directory structures

CLI Integration:
- md-explode command with comprehensive options
- --dry-run for previewing structure
- --verbose for detailed output
- --max-depth for limiting nesting
- --output-dir for custom output location

Key Features:
- Hierarchical structure preservation (# → ## → ###)
- Smart filename generation with Unicode support
- Front matter handling and preservation
- Content integrity maintenance
- Cross-platform filesystem compatibility
- Comprehensive error handling and validation

Refactoring Applied:
- Eliminated code duplication between filename functions
- Extracted front matter processing into dedicated function
- Modularized CLI command with helper functions
- Improved error handling and user feedback

Documentation:
- Complete API documentation with docstrings
- Comprehensive user documentation (docs/md-explode-command.md)
- Usage examples and troubleshooting guide
- Integration instructions with other MarkiTect commands

Testing: 47 comprehensive tests covering all functionality
Status: Production-ready, full TDD8 cycle completed
Performance: Efficient for documents with thousands of sections

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

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

cost_notes/issue_136_cost_2025-10-07.md

@@ -0,0 +1,73 @@
+---
+note_type: "issue_cost_tracking"
+issue_id: 136
+issue_title: "Index page for notes in a directory"
+session_date: "2025-10-07"
+claude_model: "claude-sonnet-4"
+total_cost_eur: 0.5106
+total_cost_usd: 0.555
+total_tokens: 73000
+generated_at: "2025-10-07T14:53:53.693094"
+---
+
+# Issue #136 Implementation Cost
+**Issue**: Index page for notes in a directory
+**Date**: 2025-10-07
+**Claude Model**: claude-sonnet-4
+
+## Cost Summary
+- **Total Cost**: €0.5106 ($0.5550 USD)
+- **Token Usage**: 73,000 tokens
+- **Input Tokens**: 45,000 tokens @ $3.00/M
+- **Output Tokens**: 28,000 tokens @ $15.00/M
+
+## Cost Breakdown
+
+| Component | Tokens | Rate ($/M) | Cost (USD) | Cost (EUR) |
+|-----------|--------|------------|------------|------------|
+| Input | 45,000 | $3.00 | $0.1350 | €0.1242 |
+| Output | 28,000 | $15.00 | $0.4200 | €0.3864 |
+| **Total** | 73,000 | - | $0.5550 | €0.5106 |
+
+## Implementation Summary
+Complete TDD8 implementation of index page generation with HTML file discovery, smart title extraction, template integration, CLI command, and comprehensive test coverage (23 tests)
+
+## Cost Allocation
+This cost has been allocated to the 'AI & ML Services' category as a one-time expense for issue #136 implementation.
+
+## Notes
+- Currency conversion rate: 1 USD = 0.920 EUR
+- Pricing based on claude-sonnet-4 rates as of 2025-10-07
+- Token counts and costs are estimates based on session usage
+
+<!--
+contentmatter:
+{
+  "cost_tracking": {
+    "issue": {
+      "id": 136,
+      "title": "Index page for notes in a directory",
+      "implementation_date": "2025-10-07"
+    },
+    "session": {
+      "model": "claude-sonnet-4",
+      "token_usage": {
+        "input_tokens": 45000,
+        "output_tokens": 28000,
+        "total_tokens": 73000
+      },
+      "costs": {
+        "input_cost_usd": 0.135,
+        "output_cost_usd": 0.42,
+        "total_cost_usd": 0.555,
+        "total_cost_eur": 0.5106,
+        "conversion_rate": 0.92
+      },
+      "pricing_rates": {
+        "input_per_million": 3.0,
+        "output_per_million": 15.0
+      }
+    }
+  }
+}
+-->