feat(views): add hide_done_categories config option (#153)

Problem: Categories where every task is done still render in the buffer,
cluttering the view when entire categories are finished.

Solution: Add `view.category.hide_done_categories` (boolean, default
false). When enabled, `category_view()` skips categories whose tasks are
all done/deleted, returns their IDs as `done_cat_hidden_ids`, and
`_on_write` merges those IDs into `hidden_ids` passed to `diff.apply()`
so hidden tasks are not mistakenly deleted on `:w`.

lua/pending/init.lua

@@ -509,6 +509,9 @@ function M._on_write(bufnr)
   if #stack > UNDO_MAX then
     table.remove(stack, 1)
   end
+  for id in pairs(buffer.done_cat_hidden_ids()) do
+    hidden[id] = true
+  end
   local new_refs = diff.apply(lines, s, hidden)
   M._recompute_counts()
   buffer.render(bufnr)