refactor: consolidate formatting logic into OutputFormatter class

- Move all formatting methods from JsonlToMarkdownConverter to OutputFormatter
- Create FormatterContext interface to encapsulate all shared state
- Update ToolResultFormatter to use injected OutputFormatter instance
- Fix filename generation to skip command elements when capturing user prompt
- Improve code organization with better separation of concerns

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