"Archive completed tasks" empties every board of what is finished into a
twin of the boards folder - `Boards/work.md` into `Boards/archive/work.md` -
itself a Kanban board with the same lanes, so the history opens like any
other board and the live one stays short. The same pass lifts a board's
native archive (everything below the `***` separator) into that mirror, so
finished work is not left sitting in two places.
Archived tasks render beside live ones when "Include the archive" and "Show
completed" are both on, from the mirrors and from any `***` section still in
a board. They carry an Archive badge instead of a project chip, ride with
the sphere they came from in the rail, and refuse drops.
