feat(init): allow :Pending done with no args to use cursor line

Problem: `:Pending done` required an explicit task ID, making it
awkward to mark the current task done while inside the pending buffer.

Solution: When called with no ID, `M.done()` reads the cursor row from
`buffer.meta()` to resolve the task ID, erroring if the cursor is not
on a saved task line.

lua/pending/init.lua

@@ -430,13 +430,28 @@ function M.toggle_complete()
   end
 end
 
----@param id_str string
+---@param id_str? string
 ---@return nil
 function M.done(id_str)
-  local id = tonumber(id_str)
-  if not id then
-    log.error('Invalid task ID: ' .. tostring(id_str))
-    return
+  local id
+  if not id_str or id_str == '' then
+    local row = vim.api.nvim_win_get_cursor(0)[1]
+    local meta = buffer.meta()
+    if not meta[row] or meta[row].type ~= 'task' then
+      log.error('Cursor is not on a task line.')
+      return
+    end
+    id = meta[row].id
+    if not id then
+      log.error('Task has no ID — save the buffer first.')
+      return
+    end
+  else
+    id = tonumber(id_str)
+    if not id then
+      log.error('Invalid task ID: ' .. tostring(id_str))
+      return
+    end
   end
   local s = get_store()
   s:load()