dev/sg/usershell: refactor for clarity, add a way to get run.Output

Created by: bobheadxi

Took a while for me to understand what kinds of things usershell.Cmd was doing, so this PR refactors the wrapping into usershell.Wrap, which clearly returns exactly what gets changed and provided, and updates usershell.Cmd to use that instead.

This also allows us to add a convenience adapter to run.Output instead.

Test plan

n/a, behaves the same