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/agent_message.py

@@ -0,0 +1,31 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict
+
+
+class MessageCreate(BaseModel):
+    from_agent: str
+    to_agent: str
+    subject: str
+    body: str
+    thread_id: uuid.UUID | None = None
+
+
+class MessageReply(BaseModel):
+    from_agent: str
+    body: str
+
+
+class MessageRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    from_agent: str
+    to_agent: str
+    subject: str
+    body: str
+    thread_id: uuid.UUID | None = None
+    read_at: datetime | None = None
+    archived_at: datetime | None = None
+    created_at: datetime