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/progress_event.py

@@ -0,0 +1,27 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, String, Text, func
+from sqlalchemy.dialects.postgresql import JSONB, UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from hub_core.models.base import Base, new_uuid
+
+
+class ProgressEvent(Base):
+    """Generic append-only event log for hub activity."""
+
+    __tablename__ = "progress_events"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=new_uuid
+    )
+    event_type: Mapped[str] = mapped_column(String(50), nullable=False, index=True)
+    summary: Mapped[str] = mapped_column(Text, nullable=False)
+    detail: Mapped[dict | None] = mapped_column(JSONB, nullable=True)
+    subject_refs: Mapped[dict | None] = mapped_column(JSONB, nullable=True)
+    author: Mapped[str | None] = mapped_column(String(100), nullable=True)
+    session_id: Mapped[str | None] = mapped_column(String(100), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), nullable=False, index=True
+    )