Learn Russian: Мы повторяем текст.

Because the action is happening to a separate direct object (текст). Reflexive endings like -ся are used when the action reflects back on the subject. If we added it here and said мы повторяемся, it would change the meaning to "we are repeating ourselves."

Since "the text" is the object being repeated, shouldn't текст change its ending?

It is actually in the Accusative case (the case for direct objects), but its ending doesn't change here. In Russian, masculine inanimate nouns like текст have an Accusative form that looks exactly the same as their basic dictionary form.

How do we get the ending -ем for the verb?

The dictionary form of the verb is повторять (to repeat). It is a standard First Conjugation verb. To match мы (we) in the present tense, you drop the -ть ending and add -ем to get повторяем.

Does this mean "We are repeating" right now, or "We repeat" as a habit?

It can be either! Russian only has one present tense, so мы повторяем translates to both "we repeat" (as a regular habit) and "we are repeating" (happening right this second) depending on the context.