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/config.lua

@@ -84,6 +84,7 @@
 ---@class pending.CategoryViewConfig
 ---@field order? string[]
 ---@field folding? boolean|pending.FoldingConfig
+---@field hide_done_categories? boolean
 
 ---@class pending.QueueViewConfig
 
@@ -130,6 +131,7 @@ local defaults = {
     category = {
       order = {},
       folding = true,
+      hide_done_categories = false,
     },
     queue = {},
   },