Add CLI tool

[Rujuu-prog]

Summary

Enable running markdown2notion directly from the command line.

Background

Currently the package can only be used as a library.
With a CLI, users could try it instantly with:

npx markdown2notion README.md --db <database-id>

This greatly lowers the entry barrier for new users.

Tasks

• Add bin field to package.json

• Implement CLI argument parsing (e.g. with yargs)

• Required args: --db, --token (or env.NOTION_TOKEN)

• Optional args: --dry-run, --update, --verbose

• Update README with Quick Start CLI usage

Acceptance criteria

Running npx markdown2notion README.md --db works as expected

Invalid arguments show help message