feat(init): multi-level priority with <C-a>/<C-x>

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).

lua/pending/buffer.lua

@@ -57,7 +57,7 @@ local function setup_syntax(bufnr)
       syntax clear
       syntax match taskId /^\/\d\+\// conceal
       syntax match taskHeader /^\S.*$/ contains=taskId
-      syntax match taskPriority /! / contained containedin=taskLine
+      syntax match taskPriority /\[\d\+\] / contained containedin=taskLine
       syntax match taskLine /^\/\d\+\/  .*$/ contains=taskId,taskPriority
     ]])
   end)