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.

lua/pending/diff.lua

@@ -65,6 +65,7 @@ function M.parse_buffer(lines)
 end
 
 ---@param lines string[]
+---@return nil
 function M.apply(lines)
   local parsed = M.parse_buffer(lines)
   local now = timestamp()