Q:

What is the grammatical term for the word for someone who does something?

What is the name for the type of word that describes a ‘doer’ of the activity?  For instance, someone who takes photos is called a photographer, someone who sculpts is a sculpter, someone who robs is a robber, someone who runs from the police is a runner, someone who hides for three days is a wanted fugitive but also a hider.
What do you call those ‘-er’ words?  Kind of similar in ways to demonyms but not quite.

A:

They are called agent nouns.

A noun denoting someone or something that performs the action of a verb, usually ending in -er or -or, e.g. worker, accelerator.
— Oxford

