feat(buffer): persist fold state across sessions

Problem: folded category headers are lost when Neovim exits because
`_fold_state` only lives in memory. Users must re-fold categories
every session.

Solution: store folded category names in the JSON data file as a
top-level `folded_categories` field. On first render, `restore_folds`
seeds from the store instead of the empty in-memory state. Folds are
persisted on `M.close()` and `VimLeavePre`.

lua/pending/buffer.lua

@@ -1,4 +1,5 @@
 local config = require('pending.config')
+local log = require('pending.log')
 local views = require('pending.views')
 
 ---@class pending.buffer
@@ -18,6 +19,8 @@ local current_view = nil
 local _meta = {}
 ---@type table<integer, table<string, boolean>>
 local _fold_state = {}
+---@type boolean
+local _initial_fold_loaded = false
 ---@type string[]
 local _filter_predicates = {}
 ---@type table<integer, true>
@@ -89,12 +92,52 @@ function M.clear_marks(b)
   vim.api.nvim_buf_clear_namespace(b or task_bufnr, task_ns, 0, -1)
 end
 
+---@return nil
+function M.persist_folds()
+  log.debug(('persist_folds: view=%s store=%s'):format(tostring(current_view), tostring(_store ~= nil)))
+  if current_view ~= 'category' or not _store then
+    log.debug('persist_folds: early return (view or store)')
+    return
+  end
+  local bufnr = task_bufnr
+  if not bufnr or not vim.api.nvim_buf_is_valid(bufnr) then
+    log.debug('persist_folds: early return (no valid bufnr)')
+    return
+  end
+  local folded = {}
+  local seen = {}
+  local wins = vim.fn.win_findbuf(bufnr)
+  log.debug(('persist_folds: checking %d windows for bufnr=%d, meta has %d entries'):format(#wins, bufnr, #_meta))
+  for _, winid in ipairs(wins) do
+    if vim.api.nvim_win_is_valid(winid) then
+      vim.api.nvim_win_call(winid, function()
+        for lnum, m in ipairs(_meta) do
+          if m.type == 'header' and m.category and not seen[m.category] then
+            local closed = vim.fn.foldclosed(lnum)
+            log.debug(('persist_folds: win=%d lnum=%d cat=%s foldclosed=%d'):format(winid, lnum, m.category, closed))
+            if closed ~= -1 then
+              seen[m.category] = true
+              table.insert(folded, m.category)
+            end
+          end
+        end
+      end)
+    end
+  end
+  log.debug(('persist_folds: saving %d folded categories: %s'):format(#folded, table.concat(folded, ', ')))
+  _store:set_folded_categories(folded)
+end
+
 ---@return nil
 function M.close()
   if not task_winid or not vim.api.nvim_win_is_valid(task_winid) then
     task_winid = nil
     return
   end
+  M.persist_folds()
+  if _store then
+    _store:save()
+  end
   local wins = vim.api.nvim_list_wins()
   if #wins == 1 then
     vim.cmd.enew()
@@ -275,7 +318,7 @@ local function snapshot_folds(bufnr)
     return
   end
   for _, winid in ipairs(vim.fn.win_findbuf(bufnr)) do
-    if _fold_state[winid] == nil then
+    if _fold_state[winid] == nil and _initial_fold_loaded then
       local state = {}
       vim.api.nvim_win_call(winid, function()
         for lnum, m in ipairs(_meta) do
@@ -292,18 +335,39 @@ local function snapshot_folds(bufnr)
 end
 
 local function restore_folds(bufnr)
+  log.debug(('restore_folds: view=%s folding_enabled=%s'):format(
+    tostring(current_view), tostring(config.resolve_folding().enabled)))
   if current_view ~= 'category' or not config.resolve_folding().enabled then
     return
   end
   for _, winid in ipairs(vim.fn.win_findbuf(bufnr)) do
     local state = _fold_state[winid]
     _fold_state[winid] = nil
+    log.debug(('restore_folds: win=%d has_fold_state=%s initial_loaded=%s has_store=%s'):format(
+      winid, tostring(state ~= nil), tostring(_initial_fold_loaded), tostring(_store ~= nil)))
+    if not state and not _initial_fold_loaded and _store then
+      _initial_fold_loaded = true
+      local cats = _store:get_folded_categories()
+      log.debug(('restore_folds: loaded %d categories from store: %s'):format(#cats, table.concat(cats, ', ')))
+      if #cats > 0 then
+        state = {}
+        for _, cat in ipairs(cats) do
+          state[cat] = true
+        end
+      end
+    end
     if state and next(state) ~= nil then
+      local applying = {}
+      for k in pairs(state) do
+        table.insert(applying, k)
+      end
+      log.debug(('restore_folds: applying folds for: %s'):format(table.concat(applying, ', ')))
       vim.api.nvim_win_call(winid, function()
         vim.cmd('normal! zx')
         local saved = vim.api.nvim_win_get_cursor(0)
         for lnum, m in ipairs(_meta) do
           if m.type == 'header' and m.category and state[m.category] then
+            log.debug(('restore_folds: folding lnum=%d cat=%s'):format(lnum, m.category))
             vim.api.nvim_win_set_cursor(0, { lnum, 0 })
             vim.cmd('normal! zc')
           end

lua/pending/init.lua

@@ -268,6 +268,18 @@ function M._setup_autocmds(bufnr)
       end
     end,
   })
+  vim.api.nvim_create_autocmd('VimLeavePre', {
+    group = group,
+    callback = function()
+      local bnr = buffer.bufnr()
+      log.debug(('VimLeavePre: bufnr=%s valid=%s'):format(
+        tostring(bnr), tostring(bnr and vim.api.nvim_buf_is_valid(bnr))))
+      if bnr and vim.api.nvim_buf_is_valid(bnr) then
+        buffer.persist_folds()
+        get_store():save()
+      end
+    end,
+  })
 end
 
 ---@param bufnr integer

lua/pending/store.lua

@@ -20,6 +20,7 @@ local config = require('pending.config')
 ---@field next_id integer
 ---@field tasks pending.Task[]
 ---@field undo pending.Task[][]
+---@field folded_categories string[]
 
 ---@class pending.Store
 ---@field path string
@@ -39,6 +40,7 @@ local function empty_data()
     next_id = 1,
     tasks = {},
     undo = {},
+    folded_categories = {},
   }
 end
 
@@ -171,6 +173,7 @@ function Store:load()
     next_id = decoded.next_id or 1,
     tasks = {},
     undo = {},
+    folded_categories = decoded.folded_categories or {},
   }
   for _, t in ipairs(decoded.tasks or {}) do
     table.insert(self._data.tasks, table_to_task(t))
@@ -199,6 +202,7 @@ function Store:save()
     next_id = self._data.next_id,
     tasks = {},
     undo = {},
+    folded_categories = self._data.folded_categories,
   }
   for _, task in ipairs(self._data.tasks) do
     table.insert(out.tasks, task_to_table(task))
@@ -371,6 +375,17 @@ function Store:set_next_id(id)
   self:data().next_id = id
 end
 
+---@return string[]
+function Store:get_folded_categories()
+  return self:data().folded_categories
+end
+
+---@param cats string[]
+---@return nil
+function Store:set_folded_categories(cats)
+  self:data().folded_categories = cats
+end
+
 ---@return nil
 function Store:unload()
   self._data = nil

spec/store_spec.lua

@@ -214,6 +214,35 @@ describe('store', function()
     end)
   end)
 
+  describe('folded_categories', function()
+    it('defaults to empty table when missing from JSON', function()
+      local path = tmpdir .. '/tasks.json'
+      local f = io.open(path, 'w')
+      f:write(vim.json.encode({
+        version = 1,
+        next_id = 1,
+        tasks = {},
+      }))
+      f:close()
+      s:load()
+      assert.are.same({}, s:get_folded_categories())
+    end)
+
+    it('round-trips folded categories through save and load', function()
+      s:set_folded_categories({ 'Work', 'Home' })
+      s:save()
+      s:load()
+      assert.are.same({ 'Work', 'Home' }, s:get_folded_categories())
+    end)
+
+    it('persists empty list', function()
+      s:set_folded_categories({})
+      s:save()
+      s:load()
+      assert.are.same({}, s:get_folded_categories())
+    end)
+  end)
+
   describe('active_tasks', function()
     it('excludes deleted tasks', function()
       s:add({ description = 'Active' })