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/agent_message.py

@@ -0,0 +1,44 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, String, Text, text
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from hub_core.models.base import Base, new_uuid
+
+
+class AgentMessage(Base):
+    __tablename__ = "agent_messages"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=new_uuid
+    )
+    from_agent: Mapped[str] = mapped_column(String(100), nullable=False)
+    to_agent: Mapped[str] = mapped_column(String(100), nullable=False, index=True)
+    subject: Mapped[str] = mapped_column(String(500), nullable=False)
+    body: Mapped[str] = mapped_column(Text, nullable=False)
+    thread_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("agent_messages.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+    read_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    archived_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=text("now()"),
+        nullable=False,
+    )
+
+    thread_root: Mapped["AgentMessage | None"] = relationship(
+        "AgentMessage",
+        remote_side="AgentMessage.id",
+        foreign_keys=[thread_id],
+        lazy="select",
+    )