feat(priority): add g<C-a> and g<C-x> visual batch priority mappings

Problem: Incrementing or decrementing priority required operating on one
task at a time with `<C-a>`/`<C-x>`, which is tedious when adjusting
multiple tasks.

Solution: Add `adjust_priority_visual(delta)` that iterates the visual
selection range, updates every task line's priority in one pass, then
re-renders once. Exposed as `increment_priority_visual()` /
`decrement_priority_visual()` with `g<C-a>` / `g<C-x>` defaults, new
`<Plug>` mappings, and config keys `priority_up_visual` /
`priority_down_visual`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lua/pending/config.lua

@@ -78,6 +78,8 @@
 ---@field move_up? string|false
 ---@field wip? string|false
 ---@field blocked? string|false
+---@field priority_up_visual? string|false
+---@field priority_down_visual? string|false
 
 ---@class pending.CategoryViewConfig
 ---@field order? string[]
@@ -157,6 +159,8 @@ local defaults = {
     blocked = 'gb',
     priority_up = '<C-a>',
     priority_down = '<C-x>',
+    priority_up_visual = 'g<C-a>',
+    priority_down_visual = 'g<C-x>',
   },
   sync = {},
   forge = {