What ingredient should I add to my cookies to increase shelf life?
No ingredient will make a cookie last 6 months. Some types of cookies last longer than others: for example, this recipe for biscotti can keep up to 3 months if kept in an airtight container in a cool place.

But again, there is no way to take a generic recipe for cookies and amend it to last this long. Something like American-style chocolate chip cookies simply has too much fat and moisture to keep. A possible alternative is to freeze cookie dough, possibly in pre-portioned scoops, and bake a fresh batch when you need them.