Create design document for deconflicting inputs and variables

[Racer159]

Describe what should be investigated or refactored

We need to create a design document that deconflicts / reconciles the concepts of variables and inputs and how those interplay with nested tasks / inclusions. These concepts are (mostly) separate right now but they intertwine in odd ways, variables are not loaded when people expect and the values are not very flexible.

Links to any relevant issues

#81
#17
#136

Additional context

Variables should be distinguished from (or simply integrated with) inputs and should be handled properly throughout the entire task chain with known/tested behavior on edge cases.