Edited by @billygriffin of GitHub CLI team:

People want to be able to change the state of a particular issue or pull request. The scope of this issue will be to close and reopen issues and pull requests.

We're calling merging out of scope for this because it's a more complex interaction and should be considered separately.

Proposed solution:

gh issue close [issue number or url]
gh issue reopen [issue number or url]
gh pr close [pr number or url]
gh pr reopen [pr number or url]

With no args, error message stating something to the effect of: include the issue/pr number or URL