feat(store): add JSON data store with CRUD operations

Problem: need persistent task storage with forward-compatible
schema and unknown field preservation.

Solution: add store module with load/save, add/update/delete,
ID allocation, and UDA round-trip preservation. Data stored at
stdpath('data')/todo/tasks.json with version 1 schema.

lua/todo/store.lua

@@ -0,0 +1,296 @@
+local config = require('todo.config')
+
+---@class todo.Task
+---@field id integer
+---@field description string
+---@field status 'pending'|'done'|'deleted'
+---@field category? string
+---@field priority integer
+---@field due? string
+---@field entry string
+---@field modified string
+---@field end? string
+---@field order integer
+---@field _extra? table<string, any>
+
+---@class todo.Data
+---@field version integer
+---@field next_id integer
+---@field tasks task.Task[]
+
+---@class todo.store
+local M = {}
+
+local SUPPORTED_VERSION = 1
+
+---@type todo.Data?
+local _data = nil
+
+---@return todo.Data
+local function empty_data()
+  return {
+    version = SUPPORTED_VERSION,
+    next_id = 1,
+    tasks = {},
+  }
+end
+
+---@param path string
+local function ensure_dir(path)
+  local dir = vim.fn.fnamemodify(path, ':h')
+  if vim.fn.isdirectory(dir) == 0 then
+    vim.fn.mkdir(dir, 'p')
+  end
+end
+
+---@return string
+local function timestamp()
+  return os.date('!%Y-%m-%dT%H:%M:%SZ')
+end
+
+---@type table<string, true>
+local known_fields = {
+  id = true,
+  description = true,
+  status = true,
+  category = true,
+  priority = true,
+  due = true,
+  entry = true,
+  modified = true,
+  ['end'] = true,
+  order = true,
+}
+
+---@param task todo.Task
+---@return table
+local function task_to_table(task)
+  local t = {
+    id = task.id,
+    description = task.description,
+    status = task.status,
+    entry = task.entry,
+    modified = task.modified,
+  }
+  if task.category then
+    t.category = task.category
+  end
+  if task.priority and task.priority ~= 0 then
+    t.priority = task.priority
+  end
+  if task.due then
+    t.due = task.due
+  end
+  if task['end'] then
+    t['end'] = task['end']
+  end
+  if task.order and task.order ~= 0 then
+    t.order = task.order
+  end
+  if task._extra then
+    for k, v in pairs(task._extra) do
+      t[k] = v
+    end
+  end
+  return t
+end
+
+---@param t table
+---@return todo.Task
+local function table_to_task(t)
+  local task = {
+    id = t.id,
+    description = t.description,
+    status = t.status or 'pending',
+    category = t.category,
+    priority = t.priority or 0,
+    due = t.due,
+    entry = t.entry,
+    modified = t.modified,
+    ['end'] = t['end'],
+    order = t.order or 0,
+    _extra = {},
+  }
+  for k, v in pairs(t) do
+    if not known_fields[k] then
+      task._extra[k] = v
+    end
+  end
+  if next(task._extra) == nil then
+    task._extra = nil
+  end
+  return task
+end
+
+---@return todo.Data
+function M.load()
+  local path = config.get().data_path
+  local f = io.open(path, 'r')
+  if not f then
+    _data = empty_data()
+    return _data
+  end
+  local content = f:read('*a')
+  f:close()
+  if content == '' then
+    _data = empty_data()
+    return _data
+  end
+  local ok, decoded = pcall(vim.json.decode, content)
+  if not ok then
+    error('todo.nvim: failed to parse ' .. path .. ': ' .. tostring(decoded))
+  end
+  if decoded.version and decoded.version > SUPPORTED_VERSION then
+    error(
+      'todo.nvim: data file version '
+        .. decoded.version
+        .. ' is newer than supported version '
+        .. SUPPORTED_VERSION
+        .. '. Please update the plugin.'
+    )
+  end
+  _data = {
+    version = decoded.version or SUPPORTED_VERSION,
+    next_id = decoded.next_id or 1,
+    tasks = {},
+  }
+  for _, t in ipairs(decoded.tasks or {}) do
+    table.insert(_data.tasks, table_to_task(t))
+  end
+  return _data
+end
+
+function M.save()
+  if not _data then
+    return
+  end
+  local path = config.get().data_path
+  ensure_dir(path)
+  local out = {
+    version = _data.version,
+    next_id = _data.next_id,
+    tasks = {},
+  }
+  for _, task in ipairs(_data.tasks) do
+    table.insert(out.tasks, task_to_table(task))
+  end
+  local encoded = vim.json.encode(out)
+  local f = io.open(path, 'w')
+  if not f then
+    error('todo.nvim: cannot write to ' .. path)
+  end
+  f:write(encoded)
+  f:close()
+end
+
+---@return todo.Data
+function M.data()
+  if not _data then
+    M.load()
+  end
+  return _data
+end
+
+---@return todo.Task[]
+function M.tasks()
+  return M.data().tasks
+end
+
+---@return todo.Task[]
+function M.active_tasks()
+  local result = {}
+  for _, task in ipairs(M.tasks()) do
+    if task.status ~= 'deleted' then
+      table.insert(result, task)
+    end
+  end
+  return result
+end
+
+---@param id integer
+---@return todo.Task?
+function M.get(id)
+  for _, task in ipairs(M.tasks()) do
+    if task.id == id then
+      return task
+    end
+  end
+  return nil
+end
+
+---@param fields { description: string, status?: string, category?: string, priority?: integer, due?: string, order?: integer, _extra?: table }
+---@return todo.Task
+function M.add(fields)
+  local data = M.data()
+  local now = timestamp()
+  local task = {
+    id = data.next_id,
+    description = fields.description,
+    status = fields.status or 'pending',
+    category = fields.category or config.get().default_category,
+    priority = fields.priority or 0,
+    due = fields.due,
+    entry = now,
+    modified = now,
+    ['end'] = nil,
+    order = fields.order or 0,
+    _extra = fields._extra,
+  }
+  data.next_id = data.next_id + 1
+  table.insert(data.tasks, task)
+  return task
+end
+
+---@param id integer
+---@param fields table<string, any>
+---@return todo.Task?
+function M.update(id, fields)
+  local task = M.get(id)
+  if not task then
+    return nil
+  end
+  local now = timestamp()
+  for k, v in pairs(fields) do
+    if k ~= 'id' and k ~= 'entry' then
+      task[k] = v
+    end
+  end
+  task.modified = now
+  if fields.status == 'done' or fields.status == 'deleted' then
+    task['end'] = task['end'] or now
+  end
+  return task
+end
+
+---@param id integer
+---@return todo.Task?
+function M.delete(id)
+  return M.update(id, { status = 'deleted', ['end'] = timestamp() })
+end
+
+---@param id integer
+---@return integer?
+function M.find_index(id)
+  for i, task in ipairs(M.tasks()) do
+    if task.id == id then
+      return i
+    end
+  end
+  return nil
+end
+
+---@param tasks todo.Task[]
+function M.replace_tasks(tasks)
+  M.data().tasks = tasks
+end
+
+---@param id integer
+function M.set_next_id(id)
+  M.data().next_id = id
+end
+
+function M.unload()
+  _data = nil
+end
+
+return M