feat: text objects and motions for the pending buffer

Problem: the pending buffer has action-button mappings but no Vim
grammar. You cannot dat to delete a task, cit to change a description,
or ]] to jump to the next category header.

Solution: add textobj.lua with at/it (a task / inner task), aC/iC
(a category / inner category), ]]/[[ (next/prev header), and ]t/[t
(next/prev task). All text objects work in operator-pending and visual
modes; motions work in normal, visual, and operator-pending. Mappings
are configurable via the keymaps table and exposed as <Plug> mappings.

lua/pending/config.lua

@@ -11,6 +11,14 @@
 ---@field undo? string|false
 ---@field open_line? string|false
 ---@field open_line_above? string|false
+---@field a_task? string|false
+---@field i_task? string|false
+---@field a_category? string|false
+---@field i_category? string|false
+---@field next_header? string|false
+---@field prev_header? string|false
+---@field next_task? string|false
+---@field prev_task? string|false
 
 ---@class pending.Config
 ---@field data_path string
@@ -47,6 +55,14 @@ local defaults = {
     undo = 'U',
     open_line = 'o',
     open_line_above = 'O',
+    a_task = 'at',
+    i_task = 'it',
+    a_category = 'aC',
+    i_category = 'iC',
+    next_header = ']]',
+    prev_header = '[[',
+    next_task = ']t',
+    prev_task = '[t',
   },
 }