Problem: <C-a>/<C-x> overrode Vim's native number increment and the
visual g<C-a>/g<C-x> variants added complexity for marginal value.
toggle_complete() left the cursor on the wrong line after re-render.
Solution: remove change_priority/change_priority_visual; add
toggle_priority() (0<->1) mapped to '!', with cursor-follow after
render matching the pattern already used in priority toggle. Add
cursor-follow to toggle_complete() for the same reason. Update plugin
plugs (priority-up/down -> priority) and add 'due'/'undo' to the
:Pending completion list. Update help text accordingly.
Problem: priority was binary (0 or 1), toggled with !, with no way
to express finer gradations or use Vim's native increment idiom.
Solution: replace toggle_priority with change_priority(delta) which
clamps to floor 0. Display format changes from '! ' to '[N] ' so any
integer level is representable. Parser updated to extract numeric
level from the [N] prefix. Visual g<C-a>/g<C-x> apply the delta to
all tasks in the selection. <Plug>(pending-priority) replaced with
<Plug>(pending-priority-up) and <Plug>(pending-priority-down).
