the metal door is a closed scenery door. metal door is east of Skylight Room. the metal door is west of Dim Lit Room. ceiling is scenery in Skylight Room. a crack is part of the ceiling. light is part of the crack. Skid mark is scenery in Skylight room.
The description of the light is "A crack in the ceiling that light is shining through."
The description of the crack is "Light shines through a small crack in the ceiling."
The description of the ceiling is "It's made of rock and has a crack in it."
Instead of looking up, say "[if player is not in Skylight Room]There is nothing up there but a rocky roof.[otherwise if player is in Skylight Room] It's made of rock and has a crack in it."
That’s because “looking up” is not a defined action in the Standard Rules (though “look up” in the sense of “look up ‘zebras’ on wikipedia” is, as the “consulting it about” action). If I recall correctly, there’s some weirdness around modifying looking up, so you could just use this quick cheat method, which is what I used in Byzantine Perspective and it didn’t seem to annoy any players:
Understand "look down" as a mistake ("[the description of the floor]").
Understand "look up" as a mistake ("[the description of the ceiling]").
To make that work properly, you’ll need a “ceiling” backdrop with the appropriate descriptions for each location:
The ceiling is a backdrop. "[if the player is in Skylight Room]It's made of rock and has a crack in it.[else if the player is in an Outside room]The sky is blue with small wispy clouds.[else]It's a plain white-painted ceiling.[end if]". The ceiling is everywhere.
Current builds of Inform understand the command “look [something]” as examining, so the command LOOK UP triggers the action “examining up” (i.e. the direction up).
I suppose you could redirect that action to the ceiling (that would catch the commands EXAMINE UP and X UP as well, if any player would get the idea to try those).
Instead of examining up in the Skylight Room: try examining the ceiling.