FR - Custom inline metadata mapping

[bea-hemm]

We can currently choose the "tag prefixes" for project and context
e.g., 'project' for [project:: myproject]
My request is to add this functionality for Completion
e.g. 'completed' instead of 'completion for - [x] submit feature request [completed:: 2026-01-14]
This feature could be used for other items as well, but 'completion/completed' is my concern for compatibility. Thanks for all your work on this already, it really rocks.