Do you mean a literary agency ? If so that is a person / firm who take on new authors and trys to get their work published , just like a musical agent does for new musical acts .
Other wise 'agency' has the same meaning as anywhere else.
since both terms [influence and intertexuality] have too many operative definitions for us to fix on one for each, we will start with the generalization that influence has to do with agency......OR Tracing influences was an essential element in the rise of the 19c historicism, developed as it was under the aegis of idealistic theories that stressed agency.
Ye Gods ! Would it help if you re read the passage(s) using substitute words ? Such as 'enabler' , or 'causation' ? Go through dictionary for synonyms of agency then substitute as you read. Eventually the fog may clear.....