feat: add export arg to export to md

[cyai]

Describe the changes you have made:

Added conversation export functionality to markdown files:

• Implemented export_to_markdown() method in OpenInterpreter class
• Added --export command-line argument to specify export at startup
• Modified main() function to handle export on KeyboardInterrupt (Ctrl+C)
• Export uses default or user-specified filename
• Exports to OS-specific default directories

Pre-Submission Checklist (optional but appreciated):

• [x] I have included relevant documentation updates (stored in /docs)
• [x] I have read docs/CONTRIBUTING.md
• [x] I have read docs/ROADMAP.md

OS Tests (optional but appreciated):

• [ ] Tested on Windows
• [x] Tested on MacOS
• [ ] Tested on Linux