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/routers/policy.py

@@ -0,0 +1,30 @@
+from collections.abc import Callable
+
+from fastapi import APIRouter, HTTPException
+
+from hub_core.schemas.policy import PolicyRead, PolicyUpdate
+
+PolicyLoader = Callable[[str], PolicyRead | None]
+PolicyUpdater = Callable[[str, str], PolicyRead]
+
+
+def create_policy_router(
+    load_policy: PolicyLoader,
+    update_policy: PolicyUpdater | None = None,
+) -> APIRouter:
+    router = APIRouter(prefix="/policy", tags=["policy"])
+
+    @router.get("/{name}", response_model=PolicyRead)
+    def get_policy(name: str) -> PolicyRead:
+        policy = load_policy(name)
+        if policy is None:
+            raise HTTPException(status_code=404, detail=f"Policy '{name}' not found")
+        return policy
+
+    if update_policy is not None:
+
+        @router.put("/{name}", response_model=PolicyRead)
+        def put_policy(name: str, body: PolicyUpdate) -> PolicyRead:
+            return update_policy(name, body.content)
+
+    return router