feat(backend): add office router and agent runtime services

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/app/services/agent_runtime/base.py

@@ -0,0 +1,37 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Any, AsyncGenerator, Protocol
+
+from app.models.conversation import Conversation, Message
+from app.models.user import User
+
+
+RuntimeName = str
+
+
+@dataclass(slots=True)
+class RuntimePreparedContext:
+    user: User
+    conversation: Conversation
+    user_message: Message
+    assistant_message: Message
+    raw_message: str
+    full_message: str
+    file_ids: list[str]
+    model_name: str | None
+    memory_context: str | None
+
+
+class ChatRuntime(Protocol):
+    name: RuntimeName
+
+    async def chat_stream(
+        self,
+        prepared: RuntimePreparedContext,
+    ) -> AsyncGenerator[dict[str, Any], None]: ...
+
+    async def chat_once(
+        self,
+        prepared: RuntimePreparedContext,
+    ) -> tuple[str, str | None]: ...