feat: add <C-a> / <C-x> keymaps for priority increment/decrement

Problem: Priority could only be cycled with `g!` (0→1→2→3→0), with no
way to directly increment or decrement.

Solution: Add `adjust_priority()` with clamping at 0 and `max_priority`,
exposed as `increment_priority()` / `decrement_priority()` on `<C-a>` /
`<C-x>`. Includes `<Plug>` mappings and vimdoc.

plugin/pending.lua

@@ -328,6 +328,14 @@ vim.keymap.set('n', '<Plug>(pending-blocked)', function()
   require('pending').toggle_status('blocked')
 end)
 
+vim.keymap.set('n', '<Plug>(pending-priority-up)', function()
+  require('pending').increment_priority()
+end)
+
+vim.keymap.set('n', '<Plug>(pending-priority-down)', function()
+  require('pending').decrement_priority()
+end)
+
 vim.keymap.set('n', '<Plug>(pending-filter)', function()
   vim.ui.input({ prompt = 'Filter: ' }, function(input)
     if input then