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/models/domain.py

@@ -0,0 +1,23 @@
+import uuid
+
+from sqlalchemy import String, Text
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from hub_core.models.base import Base, TimestampMixin, new_uuid
+
+
+class Domain(Base, TimestampMixin):
+    __tablename__ = "domains"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=new_uuid
+    )
+    slug: Mapped[str] = mapped_column(String(50), unique=True, nullable=False, index=True)
+    name: Mapped[str] = mapped_column(String(200), nullable=False)
+    description: Mapped[str | None] = mapped_column(Text, nullable=True)
+    status: Mapped[str] = mapped_column(String(20), nullable=False, default="active")
+
+    repos: Mapped[list["ManagedRepo"]] = relationship(  # noqa: F821
+        "ManagedRepo", back_populates="domain", lazy="selectin"
+    )