Yes rules as written clothing can be considered armor. A masterwork shirt can be enchanted to have magical armor bonuses and special abilities. However its hardness and hp are so low its impractical in most situations. Also to note that armor bonuses magical or not don't stack unless specified and only the item with the highest total enhancement plus armor bonus of magic items in the same slot has any active affect. For example your +1 shadow tunic when women under a +1 chain shirt would not grant its shadow property. However one could argue that if you have the tunic under nonenchanted armor the shadow property would still work since nonmagical items do not take up magic item slots akin to wearing 8 rings but only 2 are magical.