Add multi_agent_chat supervisor agent graph and supervisor prompt.

surfsense_backend/app/agents/multi_agent_chat/supervisor/__init__.py

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+"""Supervisor agent graph only; supply routing ``tools`` from ``build_supervisor_routing_tools``."""
+
+from app.agents.multi_agent_chat.supervisor.graph import build_supervisor_agent
+
+__all__ = ["build_supervisor_agent"]

surfsense_backend/app/agents/multi_agent_chat/supervisor/graph.py

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+"""Compile the supervisor agent graph (supervisor prompt + caller-supplied routing tools)."""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import app.agents.multi_agent_chat.supervisor as supervisor_pkg
+
+from langchain.agents import create_agent
+from langchain_core.language_models import BaseChatModel
+from langchain_core.tools import BaseTool
+from langgraph.types import Checkpointer
+
+from app.agents.multi_agent_chat.shared.prompt_loader import read_prompt_md
+
+
+def build_supervisor_agent(
+    llm: BaseChatModel,
+    *,
+    tools: Sequence[BaseTool],
+    checkpointer: Checkpointer | None = None,
+):
+    """Compile the supervisor **agent** (graph). ``tools`` = output of ``build_supervisor_routing_tools``."""
+    system_prompt = read_prompt_md(supervisor_pkg.__name__, "supervisor_prompt")
+    return create_agent(
+        llm,
+        system_prompt=system_prompt,
+        tools=list(tools),
+        checkpointer=checkpointer,
+    )

surfsense_backend/app/agents/multi_agent_chat/supervisor/supervisor_prompt.md

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+You are the supervisor agent. Route Gmail-related requests through the **gmail** tool and Google Calendar requests through the **calendar** tool, each with a clear task description. Answer directly when no sub-agent is needed. When sub-agents return results, combine them into one coherent reply for the user.