feat(init): multi-level undo, quickfix due, BufEnter reload

Problem: undo was single-level with shallow references; no way to
query due/overdue tasks via quickfix; two instances sharing
tasks.json would diverge silently.

Solution: replace _undo_state with _undo_states[] (cap 20, deep
copies via store.snapshot()); add M.due() which populates the
quickfix list with overdue/due-today tasks; add BufEnter autocmd
that reloads from disk when the buffer is unmodified; expand
show_help() with folds, :Pending due, relative date syntax,
PendingOverdue, and empty-input date clearing.

lua/pending/init.lua

@@ -3,11 +3,12 @@ local diff = require('pending.diff')
 local parse = require('pending.parse')
 local store = require('pending.store')
 
----@class task
+---@class pending.init
 local M = {}
 
----@type pending.Task[]?
+---@type pending.Task[][]
-local _undo_state = nil
+local _undo_states = {}
+local UNDO_MAX = 20
 
 ---@return integer bufnr
 function M.open()
@@ -27,6 +28,16 @@ function M._setup_autocmds(bufnr)
       M._on_write(bufnr)
     end,
   })
+  vim.api.nvim_create_autocmd('BufEnter', {
+    group = group,
+    buffer = bufnr,
+    callback = function()
+      if not vim.bo[bufnr].modified then
+        store.load()
+        buffer.render(bufnr)
+      end
+    end,
+  })
 end
 
 ---@param bufnr integer
@@ -55,19 +66,23 @@ end
 ---@param bufnr integer
 function M._on_write(bufnr)
   local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
-  _undo_state = store.active_tasks()
+  local snapshot = store.snapshot()
+  table.insert(_undo_states, snapshot)
+  if #_undo_states > UNDO_MAX then
+    table.remove(_undo_states, 1)
+  end
   diff.apply(lines)
   buffer.render(bufnr)
 end
 
 function M.undo_write()
-  if not _undo_state then
+  if #_undo_states == 0 then
     vim.notify('Nothing to undo.', vim.log.levels.WARN)
     return
   end
-  store.replace_tasks(_undo_state)
+  local state = table.remove(_undo_states)
+  store.replace_tasks(state)
   store.save()
-  _undo_state = nil
   buffer.render(buffer.bufnr())
 end
 
@@ -227,6 +242,49 @@ function M.archive(days)
   end
 end
 
+function M.due()
+  local today = os.date('%Y-%m-%d') --[[@as string]]
+  local bufnr = buffer.bufnr()
+  local is_valid = bufnr ~= nil and vim.api.nvim_buf_is_valid(bufnr)
+  local meta = is_valid and buffer.meta() or nil
+  local qf_items = {}
+
+  if meta and bufnr then
+    for lnum, m in ipairs(meta) do
+      if m.type == 'task' and m.raw_due and m.status ~= 'done' and m.raw_due <= today then
+        local task = store.get(m.id or 0)
+        local label = m.raw_due < today and '[OVERDUE] ' or '[DUE] '
+        table.insert(qf_items, {
+          bufnr = bufnr,
+          lnum = lnum,
+          col = 1,
+          text = label .. (task and task.description or ''),
+        })
+      end
+    end
+  else
+    store.load()
+    for _, task in ipairs(store.active_tasks()) do
+      if task.status == 'pending' and task.due and task.due <= today then
+        local label = task.due < today and '[OVERDUE] ' or '[DUE] '
+        local text = label .. task.description
+        if task.category then
+          text = text .. ' [' .. task.category .. ']'
+        end
+        table.insert(qf_items, { text = text })
+      end
+    end
+  end
+
+  if #qf_items == 0 then
+    vim.notify('No due or overdue tasks.')
+    return
+  end
+
+  vim.fn.setqflist(qf_items, 'r')
+  vim.cmd('copen')
+end
+
 function M.show_help()
   local cfg = require('pending.config').get()
   local dk = cfg.date_syntax or 'due'
@@ -239,12 +297,14 @@ function M.show_help()
     'd        Set due date',
     'U        Undo last write',
     'o / O    Add new task line',
-    'dd       Delete task (on :w)',
+    'dd       Delete task line (on :w)',
     'p / P    Paste (duplicates get new IDs)',
+    'zc / zo  Fold/unfold category (category view)',
     ':w       Save all changes',
     '',
     ':Pending add <text>          Quick-add task',
     ':Pending add Cat: <text>     Quick-add with category',
+    ':Pending due                 Show overdue/due qflist',
     ':Pending sync                Push to Google Calendar',
     ':Pending archive [days]      Purge old done tasks',
     ':Pending undo                Undo last write',
@@ -253,13 +313,20 @@ function M.show_help()
     '  ' .. dk .. ':YYYY-MM-DD    Set due date',
     '  cat:Name          Set category',
     '',
+    'Due date input:',
+    '  today, tomorrow, +Nd, mon-sun',
+    '  Empty input clears due date',
+    '',
+    'Highlights:',
+    '  PendingOverdue  overdue tasks (red)',
+    '',
     'Press q or <Esc> to close',
   }
   local buf = vim.api.nvim_create_buf(false, true)
   vim.api.nvim_buf_set_lines(buf, 0, -1, false, lines)
   vim.bo[buf].modifiable = false
   vim.bo[buf].bufhidden = 'wipe'
-  local width = 50
+  local width = 54
   local height = #lines
   local win = vim.api.nvim_open_win(buf, true, {
     relative = 'editor',
@@ -292,6 +359,8 @@ function M.command(args)
   elseif cmd == 'archive' then
     local d = rest ~= '' and tonumber(rest) or nil
     M.archive(d)
+  elseif cmd == 'due' then
+    M.due()
   elseif cmd == 'undo' then
     M.undo_write()
   else