feat(buffer): add category folds via foldexpr

Problem: category_view had no fold support, making it harder to
focus on one category in large lists.

Solution: add M.get_fold() returning '>1' for headers, '1' for task
lines, and '0' for blanks. M.render() now sets foldmethod=expr
(foldlevel=99) in category view and foldmethod=manual in priority.

lua/pending/buffer.lua

@@ -81,6 +81,22 @@ function M.get_indent()
   return 2
 end
 
+---@return string
+function M.get_fold()
+  local lnum = vim.v.lnum
+  local m = _meta[lnum]
+  if not m then
+    return '0'
+  end
+  if m.type == 'header' then
+    return '>1'
+  elseif m.type == 'task' then
+    return '1'
+  else
+    return '0'
+  end
+end
+
 ---@param bufnr integer
 ---@param line_meta pending.LineMeta[]
 local function apply_extmarks(bufnr, line_meta)
@@ -166,6 +182,18 @@ function M.render(bufnr)
 
   setup_syntax(bufnr)
   apply_extmarks(bufnr, line_meta)
+
+  for _, winid in ipairs(vim.fn.win_findbuf(bufnr)) do
+    if current_view == 'category' 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
+    else
+      vim.wo[winid].foldmethod = 'manual'
+      vim.wo[winid].foldenable = false
+    end
+  end
 end
 
 function M.toggle_view()