Same meaning. However your second, b, might be less common. Usually there is an object. I snuggled up to my dog in bed.Is there any difference between snuggle and snuggle up?
When I just want to get in bed to read a book and I want to be wrapped in a cover to feel cozy, could I say:
a I snuggled in bed.
b I snuggled up in bed.
c I was snuggled in bed.
d I was snuggled up in bed.
snuggle and snuggle up have the same meaning in these sentences?
I don't see this as passive. Who or what is the agent? "I was snuggled up by [whom?]"Yes, the active/passive distinction shows up in patterns of usage, here.
There are threads that VERY briefly discuss 'snuggle' issues.