Such a neat idea!
I had a lot of fun with this! I did feel like the levels were a bit too large though. I think puzzle games usually thrive when levels look small and simple at first but are actually complex due to their small size. Since this game revolves around grid movement and space management, tighter levels would really make this game shine I think. Walking around the whole map just to get a bucket for instance was not challenging but a little tedious unfortunately.
That said, I absolutely loved the mechanics shift from pushing coal to pulling buckets, really nice twist. (Having an undo button is also fantastic). Background music would have made this game even better :)
Overall, fantastic entry. really fun! Good job!!