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/diff.lua

@@ -34,10 +34,11 @@ function M.parse_buffer(lines)
       table.insert(result, { type = 'blank', lnum = i })
     elseif id or body then
       local stripped = body:match('^  (.+)$') or body
+      local prio_str = stripped:match('^%[(%d+)%] ')
       local priority = 0
-      if stripped:match('^! ') then
-        priority = 1
-        stripped = stripped:sub(3)
+      if prio_str then
+        priority = tonumber(prio_str)
+        stripped = stripped:sub(#prio_str + 4)
       end
       local description, metadata = parse.body(stripped)
       if description and description ~= '' then