feat: time-aware due dates, persistent undo, @return audit (#33)

* fix(plugin): allow command chaining with bar separator

Problem: :Pending|only failed because the command definition lacked the
bar attribute, causing | to be consumed as an argument.

Solution: Add bar = true to nvim_create_user_command so | is treated as
a command separator, matching fugitive's :Git behavior.

* refactor(buffer): remove opinionated window options

Problem: The plugin hardcoded number, relativenumber, wrap, spell,
signcolumn, foldcolumn, and cursorline in set_win_options, overriding
user preferences with no way to opt out.

Solution: Remove all cosmetic window options. Users who want them can
set them in after/ftplugin/pending.lua. Only conceallevel,
concealcursor, and winfixheight remain as functionally required.

* feat: time-aware due dates, persistent undo, @return audit

Problem: Due dates had no time component, the undo stack was lost on
restart and stored in a separate file, and many public functions lacked
required @return annotations.

Solution: Add YYYY-MM-DDThh:mm support across parse, views, recur,
complete, and init with time-aware overdue checks. Merge the undo stack
into the task store JSON so a single file holds all state. Add @return
nil annotations to all 27 void public functions across every module.

* feat(parse): flexible time parsing for @ suffix

Problem: the @HH:MM time suffix required zero-padded 24-hour format,
forcing users to write due:tomorrow@14:00 instead of due:tomorrow@2pm.

Solution: add normalize_time() that accepts bare hours (9, 14),
H:MM (9:30), am/pm (2pm, 9:30am, 12am), and existing HH:MM format,
normalizing all to canonical HH:MM on save.

* feat(complete): add info descriptions to omnifunc items

Problem: completion menu items had no description, making it hard to
distinguish between similar entries like date shorthands and recurrence
patterns.

Solution: return { word, info } tables from date_completions() and
recur_completions(), surfacing human-readable descriptions in the
completion popup.

* ci: format

lua/pending/store.lua

@@ -19,6 +19,7 @@ local config = require('pending.config')
 ---@field version integer
 ---@field next_id integer
 ---@field tasks pending.Task[]
+---@field undo pending.Task[][]
 
 ---@class pending.store
 local M = {}
@@ -34,6 +35,7 @@ local function empty_data()
     version = SUPPORTED_VERSION,
     next_id = 1,
     tasks = {},
+    undo = {},
   }
 end
 
@@ -165,13 +167,24 @@ function M.load()
     version = decoded.version or SUPPORTED_VERSION,
     next_id = decoded.next_id or 1,
     tasks = {},
+    undo = {},
   }
   for _, t in ipairs(decoded.tasks or {}) do
     table.insert(_data.tasks, table_to_task(t))
   end
+  for _, snapshot in ipairs(decoded.undo or {}) do
+    if type(snapshot) == 'table' then
+      local tasks = {}
+      for _, raw in ipairs(snapshot) do
+        table.insert(tasks, table_to_task(raw))
+      end
+      table.insert(_data.undo, tasks)
+    end
+  end
   return _data
 end
 
+---@return nil
 function M.save()
   if not _data then
     return
@@ -182,10 +195,18 @@ function M.save()
     version = _data.version,
     next_id = _data.next_id,
     tasks = {},
+    undo = {},
   }
   for _, task in ipairs(_data.tasks) do
     table.insert(out.tasks, task_to_table(task))
   end
+  for _, snapshot in ipairs(_data.undo) do
+    local serialized = {}
+    for _, task in ipairs(snapshot) do
+      table.insert(serialized, task_to_table(task))
+    end
+    table.insert(out.undo, serialized)
+  end
   local encoded = vim.json.encode(out)
   local tmp = path .. '.tmp'
   local f = io.open(tmp, 'w')
@@ -300,6 +321,7 @@ function M.find_index(id)
 end
 
 ---@param tasks pending.Task[]
+---@return nil
 function M.replace_tasks(tasks)
   M.data().tasks = tasks
 end
@@ -325,11 +347,24 @@ function M.snapshot()
   return result
 end
 
+---@return pending.Task[][]
+function M.undo_stack()
+  return M.data().undo
+end
+
+---@param stack pending.Task[][]
+---@return nil
+function M.set_undo_stack(stack)
+  M.data().undo = stack
+end
+
 ---@param id integer
+---@return nil
 function M.set_next_id(id)
   M.data().next_id = id
 end
 
+---@return nil
 function M.unload()
   _data = nil
 end