How to steer a research fellowship back to the main initial focus rather than the supervisor's programming needs?
This fits in the standard mold, I think, of "How do I assert myself with my advisor?"

You wrote one assertive email.  At this point you can choose between a more strongly worded (but still polite, of course) email, and asserting yourself in person (preferably in a special appointment).

Personally I would choose the latter in most cases.

Asserting oneself works best in a stair-step escalation.  Each step up must be discernably stronger than the previous, but it's best to avoid going from zero to 60, to save face.

Keep the conversation as neutral in tone and as brief as possible.