refactor: split monolithic converter into modular architecture

- Extract classes into separate files for better organization
- Create JsonlToMarkdownConverter, CommandParser, ToolResultFormatter, and OutputFormatter modules
- Move utility functions to dedicated utils.ts file
- Rename convert-claude-history.ts to index.ts as main entry point
- Fix TypeScript parameter property syntax for Node v24 compatibility
- Improve code maintainability with single responsibility per module
- Fix output filename consistency and remove extra blank lines

BREAKING CHANGE: Entry point changed from convert-claude-history.ts to index.ts

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