feat(ui/agents): add ThreePane orchestrator

Owns the SessionList sidebar, conversation snapshot fetch, run snapshot
fetch, and SSE subscription. Distributes events to Conversation and
RightPanel as props — the children are presentational.

ui/litellm-dashboard/src/app/(dashboard)/agents/components/three-pane/index.tsx

@@ -0,0 +1,139 @@
+"use client";
+
+/**
+ * ThreePane — orchestrator for the session view.
+ *
+ * Lays out:
+ *   - left:  SessionList (scoped to the active Agent)
+ *   - middle: Conversation (initial snapshot + live SSE events)
+ *   - right: RightPanel (Git / Terminal tabs)
+ *
+ * Owns the SSE subscription and the conversation snapshot fetch. Children
+ * are presentational — they receive `events` and the snapshot as props.
+ */
+import { useEffect, useState } from "react";
+import { Spin, message } from "antd";
+import SessionList from "@/app/(dashboard)/agents/components/session-list";
+import Conversation from "@/app/(dashboard)/agents/components/three-pane/conversation";
+import RightPanel from "@/app/(dashboard)/agents/components/three-pane/right-panel";
+import NewSessionDialog from "@/app/(dashboard)/agents/components/new-session-dialog";
+import { useSessionEventStream } from "@/app/(dashboard)/agents/hooks/useSessionEventStream";
+import { getCloudRun, getSessionConversation, listCloudSessions } from "@/lib/cloud-agents-client";
+import type { CloudAgentConversationMessage, CloudAgentRun, CloudAgentSession } from "@/types/cloud-agents";
+import { useRouter } from "next/navigation";
+
+interface ThreePaneProps {
+  agentId: string;
+  sessionId: string;
+  accessToken: string | null;
+}
+
+export default function ThreePane({ agentId, sessionId, accessToken }: ThreePaneProps) {
+  const router = useRouter();
+  const [sessions, setSessions] = useState<CloudAgentSession[]>([]);
+  const [sessionsLoading, setSessionsLoading] = useState(true);
+  const [activeSession, setActiveSession] = useState<CloudAgentSession | null>(null);
+  const [run, setRun] = useState<CloudAgentRun | null>(null);
+  const [initialMessages, setInitialMessages] = useState<CloudAgentConversationMessage[]>([]);
+  const [snapshotLoading, setSnapshotLoading] = useState(true);
+  const [showNewSession, setShowNewSession] = useState(false);
+
+  // Sessions sidebar
+  useEffect(() => {
+    if (!accessToken) return;
+    let cancelled = false;
+    setSessionsLoading(true);
+    listCloudSessions(accessToken, agentId)
+      .then((list) => {
+        if (cancelled) return;
+        setSessions(list);
+        setActiveSession(list.find((s) => s.session_id === sessionId) ?? null);
+      })
+      .catch((e: unknown) => {
+        if (!cancelled) {
+          const msg = e instanceof Error ? e.message : String(e);
+          message.error(`Failed to load sessions: ${msg}`);
+        }
+      })
+      .finally(() => {
+        if (!cancelled) setSessionsLoading(false);
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, [accessToken, agentId, sessionId]);
+
+  // Conversation snapshot + active run
+  useEffect(() => {
+    if (!accessToken) return;
+    let cancelled = false;
+    setSnapshotLoading(true);
+    Promise.all([
+      getSessionConversation(accessToken, sessionId),
+      activeSession?.active_run_id ? getCloudRun(accessToken, activeSession.active_run_id) : Promise.resolve(null),
+    ])
+      .then(([msgs, r]) => {
+        if (cancelled) return;
+        setInitialMessages(msgs);
+        setRun(r);
+      })
+      .catch((e: unknown) => {
+        if (!cancelled) {
+          const msg = e instanceof Error ? e.message : String(e);
+          message.error(`Failed to load conversation: ${msg}`);
+        }
+      })
+      .finally(() => {
+        if (!cancelled) setSnapshotLoading(false);
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, [accessToken, sessionId, activeSession?.active_run_id]);
+
+  const { events } = useSessionEventStream({
+    sessionId,
+    runId: activeSession?.active_run_id ?? null,
+    enabled: Boolean(activeSession?.active_run_id),
+  });
+
+  return (
+    <div className="flex h-[calc(100vh-64px)] w-full" data-testid="three-pane">
+      <aside className="w-[280px] shrink-0 border-r border-gray-200">
+        <SessionList
+          sessions={sessions}
+          loading={sessionsLoading}
+          activeSessionId={sessionId}
+          onNewSession={() => setShowNewSession(true)}
+        />
+      </aside>
+      <main className="min-w-0 flex-1 border-r border-gray-200">
+        {snapshotLoading ? (
+          <div className="flex h-full items-center justify-center">
+            <Spin />
+          </div>
+        ) : (
+          <Conversation
+            sessionId={sessionId}
+            accessToken={accessToken}
+            initialMessages={initialMessages}
+            events={events}
+          />
+        )}
+      </main>
+      <aside className="w-[420px] shrink-0">
+        <RightPanel run={run} events={events} />
+      </aside>
+      <NewSessionDialog
+        open={showNewSession}
+        agentId={agentId}
+        accessToken={accessToken}
+        onClose={() => setShowNewSession(false)}
+        onCreated={(s) => {
+          setShowNewSession(false);
+          router.push(`/agents/${agentId}/sessions/${s.session_id}`);
+        }}
+      />
+    </div>
+  );
+}