Clarify documentation about Claude Github integration

Claude on github is inherently less-supervised than claude on a developer's laptop, and so we have some extra questions about what its limitations are.

It'd be great if the docs could answer the following questions:

1. Can claude on github push directly to master on repos it has access to?
2. Can it merge PRs, or will it refuse to do that?
3. Is it possible to control who can talk to @claude on github? Do they have to have a Claude plan, or can anyone talk to claude once we have set it up using any developer account?
4. Whose plan will the usage be billed under - the user who set up the @claude integration, or the user who tagged @claude in a PR?