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/init.lua

@@ -88,6 +88,66 @@ function M._setup_buf_mappings(bufnr)
       vim.keymap.set('n', key --[[@as string]], fn, opts)
     end
   end
+
+  local textobj = require('pending.textobj')
+
+  ---@type table<string, { modes: string[], fn: fun(count: integer), visual_fn?: fun(count: integer) }>
+  local textobjs = {
+    a_task = {
+      modes = { 'o', 'x' },
+      fn = textobj.a_task,
+      visual_fn = textobj.a_task_visual,
+    },
+    i_task = {
+      modes = { 'o', 'x' },
+      fn = textobj.i_task,
+      visual_fn = textobj.i_task_visual,
+    },
+    a_category = {
+      modes = { 'o', 'x' },
+      fn = textobj.a_category,
+      visual_fn = textobj.a_category_visual,
+    },
+    i_category = {
+      modes = { 'o', 'x' },
+      fn = textobj.i_category,
+      visual_fn = textobj.i_category_visual,
+    },
+  }
+
+  for name, spec in pairs(textobjs) do
+    local key = km[name]
+    if key and key ~= false then
+      for _, mode in ipairs(spec.modes) do
+        if mode == 'x' and spec.visual_fn then
+          vim.keymap.set(mode, key --[[@as string]], function()
+            spec.visual_fn(vim.v.count1)
+          end, opts)
+        else
+          vim.keymap.set(mode, key --[[@as string]], function()
+            spec.fn(vim.v.count1)
+          end, opts)
+        end
+      end
+    end
+  end
+
+  ---@type table<string, fun(count: integer)>
+  local motions = {
+    next_header = textobj.next_header,
+    prev_header = textobj.prev_header,
+    next_task = textobj.next_task,
+    prev_task = textobj.prev_task,
+  }
+
+  for name, fn in pairs(motions) do
+    local key = km[name]
+    if key and key ~= false then
+      vim.keymap.set({ 'n', 'x', 'o' }, key --[[@as string]], function()
+        fn(vim.v.count1)
+      end, opts)
+    end
+  end
 end
 
 ---@param bufnr integer