Add hub-core package, docs, and State Hub integration scaffold

Extract the first reusable slice (models, schemas, routers, MCP, migrations)
from state-hub with INTENT/SCOPE, agent instructions, workplan, and aligned
inter_hub capability registry index.

hub_core/__init__.py

@@ -0,0 +1,5 @@
+"""Reusable primitives for FOS hub services."""
+
+__all__ = ["__version__"]
+
+__version__ = "0.1.0"