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/schemas/progress_event.py

@@ -0,0 +1,27 @@
+import uuid
+from datetime import datetime
+from typing import Any
+
+from pydantic import BaseModel, ConfigDict
+
+
+class ProgressEventCreate(BaseModel):
+    event_type: str
+    summary: str
+    detail: dict[str, Any] | None = None
+    subject_refs: dict[str, Any] | None = None
+    author: str | None = None
+    session_id: str | None = None
+
+
+class ProgressEventRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    event_type: str
+    summary: str
+    detail: dict[str, Any] | None = None
+    subject_refs: dict[str, Any] | None = None
+    author: str | None = None
+    session_id: str | None = None
+    created_at: datetime