feat(priority): add g<C-a> and g<C-x> visual batch priority mappings (#151)

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`.

lua/pending/init.lua

@@ -402,6 +402,26 @@ function M._setup_buf_mappings(bufnr)
     end
   end
 
+  ---@type table<string, fun()>
+  local visual_actions = {
+    priority_up_visual = function()
+      M.increment_priority_visual()
+    end,
+    priority_down_visual = function()
+      M.decrement_priority_visual()
+    end,
+  }
+
+  for name, fn in pairs(visual_actions) do
+    local key = km[name]
+    if key and key ~= false then
+      vim.keymap.set('x', key --[[@as string]], function()
+        vim.api.nvim_feedkeys(vim.api.nvim_replace_termcodes('<Esc>', true, false, true), 'nx', false)
+        fn()
+      end, opts)
+    end
+  end
+
   local textobj = require('pending.textobj')
 
   ---@type table<string, { modes: string[], fn: fun(count: integer), visual_fn?: fun(count: integer) }>
@@ -711,6 +731,53 @@ function M.decrement_priority()
   adjust_priority(-1)
 end
 
+---@param delta integer
+---@return nil
+local function adjust_priority_visual(delta)
+  local bufnr = buffer.bufnr()
+  if not bufnr then
+    return
+  end
+  if not require_saved() then
+    return
+  end
+  local start_row = vim.fn.line("'<")
+  local end_row = vim.fn.line("'>")
+  local cursor = vim.api.nvim_win_get_cursor(0)
+  local meta = buffer.meta()
+  local s = get_store()
+  local max = require('pending.config').get().max_priority or 3
+  local changed = false
+  for row = start_row, end_row do
+    if meta[row] and meta[row].type == 'task' and meta[row].id then
+      local task = s:get(meta[row].id)
+      if task then
+        local new_priority = math.max(0, math.min(max, task.priority + delta))
+        if new_priority ~= task.priority then
+          s:update(meta[row].id, { priority = new_priority })
+          changed = true
+        end
+      end
+    end
+  end
+  if not changed then
+    return
+  end
+  _save_and_notify()
+  buffer.render(bufnr)
+  pcall(vim.api.nvim_win_set_cursor, 0, cursor)
+end
+
+---@return nil
+function M.increment_priority_visual()
+  adjust_priority_visual(1)
+end
+
+---@return nil
+function M.decrement_priority_visual()
+  adjust_priority_visual(-1)
+end
+
 ---@return nil
 function M.prompt_date()
   local bufnr = buffer.bufnr()