feat: Enhanced Markdown Support with Bidirectional Conversion

[pchuri]

Summary

🚀 Major Feature: Enhanced Markdown Support with Bidirectional Conversion

This PR adds comprehensive markdown support to confluence-cli with:

• 🔄 Bidirectional Conversion: markdown ↔ Confluence storage format
• 📊 Native Storage Format: Converts to native Confluence elements (no HTML macros)
• ✨ Confluence Extensions: Support for [\!info], [\!warning], [\!note] admonitions
• 📋 Rich Elements: Tables, code blocks, lists, links with proper formatting
• 🧪 Comprehensive Testing: 17 passing tests with full coverage

Key Features

📖 Read Pages in Markdown Format

confluence read <pageId> --format markdown

⚡ Enhanced Conversion Methods

• markdownToStorage() - Convert markdown to Confluence storage format
• storageToMarkdown() - Convert Confluence storage to markdown
• htmlToMarkdown() - Convert HTML to clean markdown
• setupConfluenceMarkdownExtensions() - Custom markdown extensions

🎯 Confluence-Specific Extensions

[\!info]
This renders as a Confluence info macro

[\!warning]
This renders as a warning macro

```javascript
console.log("Code blocks with syntax highlighting");

Feature	Status
Tables	✅
Lists	✅

## Changes Made

- **📄 CLI Enhancement**: Added `--format markdown` to `confluence read` command
- **🔧 Core Functionality**: Enhanced `ConfluenceClient` with bidirectional conversion
- **📚 Documentation**: Updated `CONTRIBUTING.md` with markdown examples
- **🧪 Testing**: Comprehensive test suite with 17 passing tests
- **⚙️ Advanced Features**: Custom markdown-it extensions for Confluence

## Test Results

✅ **All 17 tests passing**  
✅ **Real-world page conversion working perfectly**  
✅ **Complex tables converted to clean markdown**  
✅ **No HTML artifacts remaining**

## Before vs After

**Before**: HTML macro wrapping  
**After**: Native Confluence storage elements with clean markdown output

This enhancement makes confluence-cli much more powerful for markdown workflows\!

## Test plan

- [x] All existing tests pass
- [x] New markdown conversion tests pass  
- [x] Manual testing with real Confluence pages
- [x] Complex table conversion verified
- [x] Code block syntax highlighting preserved
- [x] Admonition extensions working
- [x] Bidirectional conversion maintains data integrity

[github-actions]

🎉 This PR is included in version 1.4.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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