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/schemas/policy.py

@@ -0,0 +1,10 @@
+from pydantic import BaseModel
+
+
+class PolicyRead(BaseModel):
+    name: str
+    content: str
+
+
+class PolicyUpdate(BaseModel):
+    content: str