chore: initialize medical consultation agent demo

backend/app/schemas/agent.py

@@ -0,0 +1,16 @@
+from pydantic import BaseModel
+
+
+class AgentHelloUser(BaseModel):
+    """Hello 用户信息：返回宿主系统传入的上下文。"""
+
+    user_id: str
+    tenant_id: str | None = None
+    role: str | None = None
+
+
+class AgentHelloResponse(BaseModel):
+    """Hello 响应：返回用户上下文和 Demo 能力开关。"""
+
+    user: AgentHelloUser
+    features: dict