feat(23-03): wire useBrainstormerDefault and handleHandoff into ChatPanel

- Import useBrainstormerDefault hook and useToast context
- Auto-select general agent for new conversations (when activeConversationId is null)
- Add handleHandoff callback: calls chatApi.handoffSpec, invalidates messages cache, shows error toast on failure
- Pass onHandoff={handleHandoff} to ChatMessageList

ui/src/components/ChatPanel.tsx

@@ -1,8 +1,9 @@
-import { useState, useMemo } from "react";
+import { useState, useMemo, useEffect, useCallback } from "react";
 import { X } from "lucide-react";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { useChatPanel } from "../context/ChatPanelContext";
 import { useCompany } from "../context/CompanyContext";
+import { useToast } from "../context/ToastContext";
 import { ChatInput } from "./ChatInput";
 import { ChatConversationList } from "./ChatConversationList";
 import { ChatMessageList } from "./ChatMessageList";
@@ -13,6 +14,7 @@ import { chatApi } from "../api/chat";
 import { agentsApi } from "../api/agents";
 import { useChatMessages } from "../hooks/useChatMessages";
 import { useStreamingChat } from "../hooks/useStreamingChat";
+import { useBrainstormerDefault } from "../hooks/useBrainstormerDefault";
 import { resolveAgentFromContent } from "../lib/slash-commands";
 import type { AgentRole } from "@paperclipai/shared";
 
@@ -20,12 +22,33 @@ export function ChatPanel() {
   const { chatOpen, setChatOpen, activeConversationId, setActiveConversationId } = useChatPanel();
   const { selectedCompanyId } = useCompany();
   const queryClient = useQueryClient();
+  const { pushToast } = useToast();
   const [isSending, setIsSending] = useState(false);
   const [activeAgentId, setActiveAgentId] = useState<string | null>(null);
 
   const { messages } = useChatMessages(activeConversationId);
   const { streamingContent, isStreaming, startStream, stop } = useStreamingChat(activeConversationId);
 
+  const brainstormerDefaultId = useBrainstormerDefault();
+
+  // Auto-select brainstormer (general agent) for new conversations
+  useEffect(() => {
+    if (activeAgentId === null && brainstormerDefaultId !== null && !activeConversationId) {
+      setActiveAgentId(brainstormerDefaultId);
+    }
+  }, [activeAgentId, brainstormerDefaultId, activeConversationId]);
+
+  // Handoff callback: call handoff API and invalidate messages cache
+  const handleHandoff = useCallback(async (spec: { what: string; why: string; constraints: string; success: string }) => {
+    if (!activeConversationId) return;
+    try {
+      await chatApi.handoffSpec(activeConversationId, spec, "pm");
+      queryClient.invalidateQueries({ queryKey: ["chat", "messages", activeConversationId] });
+    } catch {
+      pushToast({ title: "Could not send to PM. Try again.", tone: "error" });
+    }
+  }, [activeConversationId, queryClient, pushToast]);
+
   // Load agents for routing and identity
   const { data: agents = [], isLoading: agentsLoading } = useQuery({
     queryKey: ["agents", selectedCompanyId],
@@ -179,6 +202,7 @@ export function ChatPanel() {
                 streamingAgentRole={streamingAgent?.role ?? null}
                 onEdit={handleEdit}
                 onRetry={handleRetry}
+                onHandoff={handleHandoff}
                 agentMap={agentMap}
               />
             ) : (