I thought that a grader would solely be responsible for grading homework.
Then it appears you misunderstood the expectations of your job.
Job titles like "grader" are loose and don't necessarily describe the complete set of duties expected of a person holding that role. (A "professor" does much more than just "professing"!) It's certainly very common for a student to be hired for a job which includes both grading and helping students with questions, and sometimes other duties besides. It seems that at your school, this job is simply called "grader" for convenience. The assumption is that both grading and helping students require a certain level of expertise with the course material, so somebody qualified for one should also be qualified for the other.
If they assigned you to spend time in the skills center, the implicit expectation is that you are to spend this time assisting students who come to the center looking for help. In particular, you should not plan to spend this time working on grading (unless no students show up). Grading time would be on top of this and you should plan accordingly. So no, there isn't any kind of "protection" from doing what is simply part of your job.
Assisting students at the center may also be one of the duties of a TA, but that doesn't mean that you can't be asked to do it too. It seems clear that there's plenty of this work to go around.
Note that assisting doesn't necessarily mean answering questions - if a student just asks you "what's the answer to problem 6", you should not answer that question directly! But you should work with the student to help them learn what they need in order to solve the problem on their own. There is a fine line between helping and giving away answers, and you'll get better at it with experience. You can also talk with the professor or TAs to get some guidance.
If you really don't like this task, you have the right to quit your job (unless you have some sort of binding contract). But many people find working with students to be valuable experience, especially if you have any interest in a career involving teaching. It also helps develop your communication skills, and tends to help deepen your understanding of the topic of the course.
(If you have some sort of written contract that specifies that a grader is required only to grade homework/exams and nothing else, then you might have some remedies through HR or a union or some similar structure. But I doubt that this is the case.)