If you are on a Claude Pro/Max plan, can you use it by signing into your account? Or is it API only for now?

No, I wish. That would be a really cool functionality but to my knowledge it is not possible BUT I could be wrong and would be more than happy to implement that support if someone gives me the information needed to integrate.

Perhaps an interface that allows access to the structure through skills? or an MCP?

Check out how opencode does it