create a generic settlement system

create a minimum settlement system that work as a foundation for all future settlements

requirments:
- agents of a settlement is auto spawned, and are tied to a settlement (behaviour like wander, factions etc).
- artifact/item drop is integrated with settlement
- settlements supports drag and drop into the real world

No files, tests, or entry points are named. Start by locating the existing settlement, agent, item-drop, and world interaction systems, then clarify how they should connect. Done means the minimum generic settlement foundation supports spawned, settlement-tied agents, integrated item drops, and drag-and-drop into the world.

Written by the indexing model from the issue text.