0.0000001 armor class bonus.
A teeshirt will only protect you from flower petals, nothing more.
And it will certainly not speed regeneration process.
Actually, a couple layers of cloth is suppose to be pretty good protection against slashing (certainly much better than skin). I have not gotten into a knife fight to test this, though.
There are fabrics now designed to draw sweat away from your skin and keep you cooler, so an endurance bonus would not be unreasonable. If slashing/piercing/crushing types of damage are separate in Riftrunner, light armour could provide minor protection against slashing and fire/cold. Presumably, there could not be as much enchantment on light armour as heavy.
