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/database.py

@@ -0,0 +1,18 @@
+from collections.abc import AsyncGenerator
+
+from sqlalchemy.ext.asyncio import AsyncEngine, AsyncSession, async_sessionmaker, create_async_engine
+
+
+def make_engine(database_url: str, **kwargs: object) -> AsyncEngine:
+    return create_async_engine(database_url, **kwargs)
+
+
+def make_session_factory(engine: AsyncEngine) -> async_sessionmaker[AsyncSession]:
+    return async_sessionmaker(engine, expire_on_commit=False)
+
+
+async def session_from_factory(
+    factory: async_sessionmaker[AsyncSession],
+) -> AsyncGenerator[AsyncSession, None]:
+    async with factory() as session:
+        yield session