SRD:
• One vest, vestment, or shirt on the torso
• One robe or suit of armor on the body (over a vest, vestment, or shirt)
So, a shirt/vest is not armor - for "Armor Bonus (Enchantment)" purposes. Meaning, you cannot add "+1 armor enchantment" to a shirt/vest.
The above is the only source I can find that shows a Robe is a piece of armor with armor bonus = 0. Since Robe is not listed in the armor charts, the charts seem to contradict this. My conclusion is that RAW, the Robe counts as armor, and allows "Armor Bonus (Enchantment)" to be added, but a given DM could count the item as "light armor" or "no armor". But, read on...
Bracers of Armor exist, it's logical to assume that one could place "Armor Bonus" on other items, but if the Body Slot affinity is not "Combat", might have to pay the premium pricing for using a non-normal slot for such a bonus. So, you could make something similar to Bracers of Armor, but in a shirt. Remember, this is not the same as "Armor Bonus (Enchancement)". You could wear a Shirt which provides a +5 Armor Bonus and you are un-armorered. (Just like wearing bracers of +5 armor, you are un-armored.)
One could theoretically put the same enchantment from the bracers on a suit of leather armor with Craft Wondrous Item - but the bonuses don't stack, because they are both "armor bonuses", you just use the higher bonus. In this case, you could have Leather Armor +3 (Craft Arms and Armor), enchanted with a +5 Armor bonus (Craft Wondrous Item). There is a Gold piece premium in the cost for multiple effects. Inside an anti-magic field the wearer gets the armor bonus of the Leather, in normal areas the wearer gets +5 armor bonus and +3 armor bonus (enchancement), for a total of +8 AC.
The next step is the Robe + 3, with +5 armor bonus. Inside Anti-magic areas, AC bonus is 0. In normal areas, it's +8 AC. Whether this is light armor, or no armor, is up to your DM, since it's not in the rules whether a Robe is light or counts as no armor.
Since the Armor Bonus of a Robe is 0 inside an antimagic area, I would say it's not armor. I certainly would not penalize a monk by saying he is "armored", for wearing a Scholar's Outfit or a Robe of Eyes.