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/views.lua

@@ -138,6 +138,7 @@ end
 ---@param tasks pending.Task[]
 ---@return string[] lines
 ---@return pending.LineMeta[] meta
+---@return table<integer, true> done_cat_hidden_ids
 function M.category_view(tasks)
   local by_cat = {}
   local cat_order = {}
@@ -177,6 +178,9 @@ function M.category_view(tasks)
     cat_order = ordered
   end
 
+  local hide_done = config.get().view.category.hide_done_categories
+  local done_cat_hidden = {} ---@type table<integer, true>
+
   for _, cat in ipairs(cat_order) do
     sort_tasks(by_cat[cat])
     sort_tasks(done_by_cat[cat])
@@ -184,12 +188,21 @@ function M.category_view(tasks)
 
   local lines = {}
   local meta = {}
+  local rendered = 0
 
-  for i, cat in ipairs(cat_order) do
-    if i > 1 then
+  for _, cat in ipairs(cat_order) do
+    if hide_done and #by_cat[cat] == 0 and #done_by_cat[cat] > 0 then
+      for _, t in ipairs(done_by_cat[cat]) do
+        done_cat_hidden[t.id] = true
+      end
+      goto next_cat
+    end
+
+    if rendered > 0 then
       table.insert(lines, '')
       table.insert(meta, { type = 'blank' })
     end
+    rendered = rendered + 1
     table.insert(lines, '# ' .. cat)
     table.insert(meta, { type = 'header', category = cat })
 
@@ -220,9 +233,11 @@ function M.category_view(tasks)
         forge_spans = compute_forge_spans(task, prefix_len),
       })
     end
+
+    ::next_cat::
   end
 
-  return lines, meta
+  return lines, meta, done_cat_hidden
 end
 
 ---@param tasks pending.Task[]