Sub-Workflow variables should be shown when in a sub workflow

When in a sub workflow, like a for loop, the variable available in that context should be indicated. This helps _significantly_ with discovery & UX.

No files or tests are named. Start by locating the UI that displays available workflow variables and the handling for sub-workflow contexts such as for loops. Done means variables available in the current sub-workflow context are visibly indicated for discovery.

Written by the indexing model from the issue text.