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/migrations/env.py

@@ -0,0 +1,49 @@
+import os
+from logging.config import fileConfig
+
+from alembic import context
+from sqlalchemy import engine_from_config, pool
+
+from hub_core.models import Base
+
+config = context.config
+
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+target_metadata = Base.metadata
+
+db_url = os.environ.get("DATABASE_URL")
+if db_url:
+    sync_url = db_url.replace("postgresql+asyncpg://", "postgresql+psycopg2://")
+    config.set_main_option("sqlalchemy.url", sync_url)
+
+
+def run_migrations_offline() -> None:
+    url = config.get_main_option("sqlalchemy.url")
+    context.configure(
+        url=url,
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def run_migrations_online() -> None:
+    connectable = engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+    with connectable.connect() as connection:
+        context.configure(connection=connection, target_metadata=target_metadata)
+        with context.begin_transaction():
+            context.run_migrations()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()