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/init.lua

@@ -363,6 +363,12 @@ function M._setup_buf_mappings(bufnr)
     blocked = function()
       M.toggle_status('blocked')
     end,
+    priority_up = function()
+      M.increment_priority()
+    end,
+    priority_down = function()
+      M.decrement_priority()
+    end,
     undo = function()
       M.undo_write()
     end,
@@ -646,6 +652,56 @@ function M.toggle_priority()
   end
 end
 
+---@param delta integer
+---@return nil
+local function adjust_priority(delta)
+  local bufnr = buffer.bufnr()
+  if not bufnr then
+    return
+  end
+  if not require_saved() then
+    return
+  end
+  local row = vim.api.nvim_win_get_cursor(0)[1]
+  local meta = buffer.meta()
+  if not meta[row] or meta[row].type ~= 'task' then
+    return
+  end
+  local id = meta[row].id
+  if not id then
+    return
+  end
+  local s = get_store()
+  local task = s:get(id)
+  if not task then
+    return
+  end
+  local max = require('pending.config').get().max_priority or 3
+  local new_priority = math.max(0, math.min(max, task.priority + delta))
+  if new_priority == task.priority then
+    return
+  end
+  s:update(id, { priority = new_priority })
+  _save_and_notify()
+  buffer.render(bufnr)
+  for lnum, m in ipairs(buffer.meta()) do
+    if m.id == id then
+      vim.api.nvim_win_set_cursor(0, { lnum, 0 })
+      break
+    end
+  end
+end
+
+---@return nil
+function M.increment_priority()
+  adjust_priority(1)
+end
+
+---@return nil
+function M.decrement_priority()
+  adjust_priority(-1)
+end
+
 ---@return nil
 function M.prompt_date()
   local bufnr = buffer.bufnr()