feat(buffer): add configurable category-level folds

Problem: category folds were hardcoded with no config option, no custom
foldtext, and no vimdoc coverage.

Solution: add `folding` config field (boolean or table with `foldtext`
format string). Default foldtext is `%c (%n tasks)` with automatic
singular/plural. Gate all fold logic on the config so `folding = false`
disables folds entirely. Document the new option in vimdoc.

lua/pending/config.lua

@@ -1,3 +1,10 @@
+---@class pending.FoldingConfig
+---@field foldtext? string|false
+
+---@class pending.ResolvedFolding
+---@field enabled boolean
+---@field foldtext string|false
+
 ---@class pending.Icons
 ---@field pending string
 ---@field done string
@@ -55,6 +62,7 @@
 ---@field drawer_height? integer
 ---@field debug? boolean
 ---@field keymaps pending.Keymaps
+---@field folding? boolean|pending.FoldingConfig
 ---@field sync? pending.SyncConfig
 ---@field icons pending.Icons
 
@@ -70,6 +78,7 @@ local defaults = {
   date_syntax = 'due',
   recur_syntax = 'rec',
   someday_date = '9999-12-30',
+  folding = true,
   category_order = {},
   keymaps = {
     close = 'q',
@@ -119,4 +128,15 @@ function M.reset()
   _resolved = nil
 end
 
+---@return pending.ResolvedFolding
+function M.resolve_folding()
+  local raw = M.get().folding
+  if raw == false then
+    return { enabled = false, foldtext = false }
+  elseif raw == true or raw == nil then
+    return { enabled = true, foldtext = '%c (%n tasks)' }
+  end
+  return { enabled = true, foldtext = raw.foldtext or false }
+end
+
 return M