feat: Sync files across repo

[HugoRCD]

This feature is the first genuine GitHub capability aimed at enabling direct file uploads in Shelve, ensuring that one or multiple files stay up to date across various repositories. This feature will also function with the CLI following an upgrade of the shelve config file, which may look something like this:

# shelve.config.ts
export default {
  ecosystem: {
    name: 'my-org',
    repos: ['repo1', 'repo2'],
    shared: {
      files: ['CONTRIBUTING.md', 'CODE_OF_CONDUCT.md'],
      templates: './templates',
      branding: './branding'
    }
  },
  markdown: {
    templates: {
      readme: './templates/README.md',
      contributing: './templates/CONTRIBUTING.md'
    },
    autoUpdate: {
      contributors: true,
      dependencies: true,
      stats: true
    }
  }
}