My question is simple: does raising the dead with a "raise dead" spell constitute a type of summoning that can be affected by the augment summoning feat? (Aren't we effectively "summoning" the dead spirit from a different plane?) I suspect it is not, but want to double-check on behalf of one of my players.
No, it does not.
Augment Summoning refers to Conjuration (Summoning) spells only. See the text:
They could be more explicit and state Conjuration (Summoning), but ultimately it’s pretty clear that this is what they meant. Technically, “summoning” is a particular form of calling another creature or spirit that follows the specific rules given under the Conjuration (Summoning) section of the Magic Overview. Raise dead is not a Conjuration (Summoning) spell, it is Conjuration (Healing).
Moreover, this strikes me as a bad idea in any event. Dying shouldn’t be a good thing...