The smallest footprint for an actual scripting probably will be posix sh - since you already have it ready.
A slightly bigger footprint would be Python or Lua.
If you can drop your requirement for actual scripting and are willing to add a compile step, Go and it’s ecosystem is pretty dang powerful and it’s really easy to learn for small automation tasks.
Personally, with the requirement of not adding too much space for runtimes, I’d write it in go. You don’t need a runtime, you can compile it to a really small zero dependency lib and you have clean and readable code that you can extend, test and maintain easily.