Fix session_manager imports - define Operation/Submission locally

Operation and Submission dataclasses are defined in agent/main.py,
not exported from agent.core.session. Define them locally in
session_manager.py to match the expected structure.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/session_manager.py

@@ -5,14 +5,31 @@ import logging
 import uuid
 from dataclasses import dataclass, field
 from datetime import datetime
-from typing import Any
+from typing import Any, Optional
 
 from agent.config import load_config
 from agent.core.agent_loop import process_submission
-from agent.core.session import Event, Operation, OpType, Session, Submission
+from agent.core.session import Event, OpType, Session
 from agent.core.tools import ToolRouter
 from websocket import manager as ws_manager
 
+
+# These dataclasses match agent/main.py structure
+@dataclass
+class Operation:
+    """Operation to be executed by the agent."""
+
+    op_type: OpType
+    data: Optional[dict[str, Any]] = None
+
+
+@dataclass
+class Submission:
+    """Submission to the agent loop."""
+
+    id: str
+    operation: Operation
+
 logger = logging.getLogger(__name__)