feat(buffer): add configurable category-level folds (#91)

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/buffer.lua

@@ -236,8 +236,30 @@ local function setup_highlights()
   vim.api.nvim_set_hl(0, 'PendingFilter', { link = 'DiagnosticWarn', default = true })
 end
 
+---@return string
+function M.get_foldtext()
+  local folding = config.resolve_folding()
+  if not folding.foldtext then
+    return vim.fn.foldtext()
+  end
+  local line = vim.fn.getline(vim.v.foldstart)
+  local cat = line:match('^#%s+(.+)$') or line
+  local task_count = vim.v.foldend - vim.v.foldstart
+  local icons = config.get().icons
+  local result = folding.foldtext
+    :gsub('%%c', cat)
+    :gsub('%%n', tostring(task_count))
+    :gsub('(%d+) (%w+)s%)', function(n, word)
+      if n == '1' then
+        return n .. ' ' .. word .. ')'
+      end
+      return n .. ' ' .. word .. 's)'
+    end)
+  return icons.category .. ' ' .. result
+end
+
 local function snapshot_folds(bufnr)
-  if current_view ~= 'category' then
+  if current_view ~= 'category' or not config.resolve_folding().enabled then
     return
   end
   for _, winid in ipairs(vim.fn.win_findbuf(bufnr)) do
@@ -256,7 +278,7 @@ local function snapshot_folds(bufnr)
 end
 
 local function restore_folds(bufnr)
-  if current_view ~= 'category' then
+  if current_view ~= 'category' or not config.resolve_folding().enabled then
     return
   end
   for _, winid in ipairs(vim.fn.win_findbuf(bufnr)) do
@@ -328,12 +350,18 @@ function M.render(bufnr)
   setup_syntax(bufnr)
   apply_extmarks(bufnr, line_meta)
 
+  local folding = config.resolve_folding()
   for _, winid in ipairs(vim.fn.win_findbuf(bufnr)) do
-    if current_view == 'category' then
+    if current_view == 'category' and folding.enabled then
       vim.wo[winid].foldmethod = 'expr'
       vim.wo[winid].foldexpr = 'v:lua.require("pending.buffer").get_fold()'
       vim.wo[winid].foldlevel = 99
       vim.wo[winid].foldenable = true
+      if folding.foldtext then
+        vim.wo[winid].foldtext = 'v:lua.require("pending.buffer").get_foldtext()'
+      else
+        vim.wo[winid].foldtext = 'foldtext()'
+      end
     else
       vim.wo[winid].foldmethod = 'manual'
       vim.wo[winid].foldenable = false

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