In English most verbs have a form that describes the person that performs the action. Some examples would be Runner, Climber, or Jumper. What are these kinds of words called? Are the called the same thing as words like Guitarist, Pianist, or Scriptorian?
Whether guitarist or pianist are agent nouns, depends on your definition. At any rate, in these cases the suffix -ist creates nouns from other nouns, not from verbs, so morphologically there is no action it is the agent of. In addition, in the case of pianist, the noun wasn't even formed in English, but was borrowed as a whole from French. So in English, there is no suffix in it at all. The whole word is the root.
Scriptorian is not a noun I have heard before or was able to find in any dictionary.