feat(ui/agents): add SessionList sidebar

Vertical list of SessionRow under the active agent with a + New action
pinned to the header. Shared by /agents/{aid} and the three-pane view.

ui/litellm-dashboard/src/app/(dashboard)/agents/components/session-list.tsx

@@ -0,0 +1,45 @@
+"use client";
+
+/**
+ * SessionList — left sidebar showing all sessions belonging to the active
+ * Agent. Used in both the per-agent landing page and the three-pane view.
+ *
+ * Rendered as a vertical list of SessionRow with a "+ New Session" action
+ * pinned at the top.
+ */
+import { Button, Empty, Spin, Typography } from "antd";
+import SessionRow from "@/app/(dashboard)/agents/components/session-row";
+import type { CloudAgentSession } from "@/types/cloud-agents";
+
+const { Text } = Typography;
+
+interface SessionListProps {
+  sessions: CloudAgentSession[];
+  loading: boolean;
+  activeSessionId?: string | null;
+  onNewSession: () => void;
+}
+
+export default function SessionList({ sessions, loading, activeSessionId, onNewSession }: SessionListProps) {
+  return (
+    <div className="flex h-full flex-col" data-testid="session-list">
+      <div className="flex items-center justify-between border-b border-gray-200 px-3 py-2">
+        <Text strong>Sessions</Text>
+        <Button size="small" type="primary" onClick={onNewSession} data-testid="new-session-btn">
+          + New
+        </Button>
+      </div>
+      <div className="min-h-0 flex-1 overflow-y-auto">
+        {loading ? (
+          <div className="flex items-center justify-center p-4">
+            <Spin size="small" />
+          </div>
+        ) : sessions.length === 0 ? (
+          <Empty description="No sessions yet" className="!my-8" />
+        ) : (
+          sessions.map((s) => <SessionRow key={s.session_id} session={s} active={s.session_id === activeSessionId} />)
+        )}
+      </div>
+    </div>
+  );
+}