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.

doc/pending.txt

@@ -38,7 +38,7 @@ Features: ~
 - Multi-level undo (up to 20 `:w` saves, persisted across sessions)
 - Quick-add from the command line with `:Pending add`
 - Quickfix list of overdue/due-today tasks via `:Pending due`
-- Foldable category sections (`zc`/`zo`) in category view
+- Configurable category folds (`zc`/`zo`) with custom foldtext
 - Omnifunc completion for `cat:`, `due:`, and `rec:` tokens (`<C-x><C-o>`)
 - Google Calendar one-way push via OAuth PKCE
 - Google Tasks bidirectional sync via OAuth PKCE
@@ -274,8 +274,8 @@ Default buffer-local keys: ~
     `U`           Undo the last `:w` save (`undo`)
     `o`           Insert a new task line below (`open_line`)
     `O`           Insert a new task line above (`open_line_above`)
-    `zc`          Fold the current category section (category view only)
-    `zo`          Unfold the current category section (category view only)
+    `zc`          Fold the current category section (requires `folding`)
+    `zo`          Unfold the current category section (requires `folding`)
 
 Text objects (operator-pending and visual): ~
 
@@ -595,6 +595,7 @@ loads: >lua
       date_syntax = 'due',
       recur_syntax = 'rec',
       someday_date = '9999-12-30',
+      folding = true,
       category_order = {},
       keymaps = {
         close = 'q',
@@ -684,6 +685,35 @@ Fields: ~
                          given order. Categories not in the list are appended
                          after the ordered ones in their natural order.
 
+    {folding}            (boolean|table, default: true)  *pending.FoldingConfig*
+                         Controls category-level folds in category view. When
+                         `true`, folds are enabled with the default foldtext
+                         `'%c (%n tasks)'`. When `false`, folds are disabled
+                         entirely. When a table, folds are enabled and the
+                         table may contain:
+
+                           {foldtext}  (string|false, default: '%c (%n tasks)')
+                                       Custom foldtext format string. Set to
+                                       `false` to use Vim's built-in
+                                       foldtext. Two specifiers are
+                                       available:
+                                         `%c`  category name
+                                         `%n`  number of tasks in the fold
+                                       The category icon is prepended
+                                       automatically. When `false`, the
+                                       default Vim foldtext is used.
+
+                         Folds only apply to category view; priority view
+                         is always fold-free regardless of this setting.
+
+                         Examples: >lua
+                             vim.g.pending = { folding = true }
+                             vim.g.pending = { folding = false }
+                             vim.g.pending = {
+                               folding = { foldtext = '%c (%n tasks)' },
+                             }
+<
+
     {keymaps}            (table, default: see below)       *pending.Keymaps*
                          Buffer-local key bindings. Each field maps an action
                          name to a key string. Set a field to `false` to