go-instagram/instagram ? reverse-engineered prompt

Build me a small Go library that can read public Instagram content from the web, without any third party packages or CGO. I want a simple API where I can create a client, optionally pass a session cookie if needed, and then fetch a user profile, their posts, who they follow, and their feed.

Please make it work best effort, since Instagram can be flaky and may block requests sometimes. If a request fails because of login, rate limits, or blocked access, surface that clearly so it is easy to handle in my app. Keep the API small and easy to use, with sensible defaults, but let me override the base URL, user agent, and HTTP client if I need to.

Also include a few basic tests and a README example showing how to fetch a profile and print the username, follower count, and post links.