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.

lua/pending/config.lua

@@ -137,6 +137,8 @@ local defaults = {
     move_up = 'K',
     wip = 'gw',
     blocked = 'gb',
+    priority_up = '<C-a>',
+    priority_down = '<C-x>',
   },
   sync = {},
   icons = {