trying to path through a shk can set the fully_explored flag

[doy]

Either need to do an additional pathing check that ignores monsters, or need to clear the fully_explored flag when a monster moves.

On a related note, this could also come up if there are doors in the way, but it never actually comes up because if the Doors behavior is selected, Explore is skipped entirely. We should avoid relying on this kind of thing though - skipping behaviors is supposed to just be an optimization. The issue here is that this part of the AI is stateful... should think about that.