Agent config to avoid pass-through variables relating to global config #1636

@copilot rather than accepting backend as an argument here, you should just access it via the global usethis_config object.

Originally posted by @nathanjmcdougall in #1601

We should aim to avoid having variables as inputs to function is they are relying on attributes of the usethis_config global state object. For example, rather than having a function accepting backend as an argument, we should access it via usethis_config.backend in the function, and simplify the function interface. This helps avoid pass-through variable patterns.

We should update the agent skills to explain this principle.