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/views.lua

@@ -9,6 +9,7 @@ local config = require('pending.config')
 ---@field category? string
 ---@field overdue? boolean
 ---@field show_category? boolean
+---@field priority? integer
 
 ---@class pending.views
 local M = {}
@@ -138,7 +139,7 @@ function M.category_view(tasks)
     for _, task in ipairs(all) do
       local prefix = '/' .. task.id .. '/'
       local indent = '  '
-      local prio = task.priority == 1 and '! ' or ''
+      local prio = task.priority > 0 and ('[' .. task.priority .. '] ') or ''
       local line = prefix .. indent .. prio .. task.description
       table.insert(lines, line)
       table.insert(meta, {