fix: implement link style conversion in convert command

[Mearman]

Summary

Fixes the convert command's link style conversion functionality by implementing the previously empty convertLinkStyle method. This resolves the issue where markmv convert --link-style combined would report "No changes needed" even when there were standard markdown links that should be converted.

Problem

The convertLinkStyle method in LinkConverter was just a placeholder that always returned false, meaning no link style conversions were ever performed. This caused the convert command to incorrectly report "No changes needed" for all link style conversion requests.

Solution

Key Changes

1. Implemented full convertLinkStyle method with support for all link formats:

   • markdown: Standard [text](url) format
   • combined: Combined [@url](url) format
   • claude: Claude import @url format
   • wikilink: Obsidian [[url]] format

2. Added link style detection to determine current format and avoid double-conversion

3. Added proper AST manipulation for each conversion type with appropriate node transformations

4. Added conversion validation to only convert internal links and prevent unnecessary changes

Implementation Details

• detectCurrentLinkStyle(): Identifies current link format based on text content and node structure
• convertToCombined(): Converts standard markdown to combined format [@url](url)
• convertToClaude(): Converts to Claude import format @url
• convertToWikilink(): Converts to Obsidian wikilink format [[url]]
• convertToMarkdown(): Converts back to standard markdown format

Testing

✅ Before Fix:

$ markmv convert test.md --link-style combined --verbose
No changes needed in test.md
Files modified: 0

✅ After Fix:

$ markmv convert test.md --link-style combined --verbose
Converted 4 links in test.md
Files modified: 1

✅ Conversion Example:

# Before
- [Backend CLAUDE.md](./backend/CLAUDE.md)
- [Frontend CLAUDE.md](./frontend/CLAUDE.md)

# After  
- [@./backend/CLAUDE.md](./backend/CLAUDE.md)
- [@./frontend/CLAUDE.md](./frontend/CLAUDE.md)

✅ No Double-Conversion:
Running the same command twice correctly reports "No changes needed" on the second run.

Breaking Changes

None - this fixes existing functionality without changing the API.

Test Plan

• Standard markdown to combined format conversion
• External links are ignored (not converted)
• No double-conversion when run multiple times
• Dry-run mode works correctly
• Verbose output shows correct change counts
• All link styles compile and build successfully

Related Issues

Resolves #32: markmv convert --link-style combined reports "No changes needed" for standard markdown links