Allow subcontext to be added directly and easier conversion
Follow up to:
#3573715
## Problem/motivation
* Creating a sub-context directly from the "Add" action should be possible without first creating a parent and then nesting. Site builders should be able to choose "Add as sub-context of \[parent\]" at creation time.
* Converting between parent and sub-context should be straightforward. A Context Item promoted from sub-context to parent (or demoted from parent to sub-context) should not require deletion and recreation. Suggest exposing this as an operation in the entity edit form or in the row-level Operations dropdown on the list view.
## Solution
TBD
## Tasks
- Discuss and decide whether to proceed
- If so, create a mockup and/or document the process
## AI usage disclosures
- [ ] AI assisted issue
- [ ] AI assisted code
- [ ] AI generated code
issue