Can someone please tell me what links up a summoned creature with its caster so that it's allied with and follows the caster? I'm looking through all the summon spells and summoned creatures, and none of them appear to have any scripts, factions, relationships, or other attributes on them that would create that kind of behavior.

It might not be a script or changable attribute. It might be simply that a creature summoned by a "Summon Creature" effect archetype is automatically tied to the caster as a follower (of sorts, not a proper follower with follower commands but just another actor that follows the summoner and defends him).

I think you're right, based on my observations on some other spells. For example, the "command" type spells not only have no script attached, but also no forms associated with them, so all the functionality must be encapsulated by the effect archetype.