Desalater: bump, anyone know about flashback items? :)
Taleroth: Yes, you keep it. But it might put you over weight before you can offload it.
As already mentioned by Taleroth, you do keep everything. But everything can weigh well above 300 (I was at 351, but I think the exact loot is random), so even with Strong Back you'll be overweight.
To clarify - you end Prologue with only items from the dungeon escape, but start Chapter 1 with everything you looted in the whole Prologue, including flashbacks.