feat: Introduce session context and add session duration stat for /stats command (#854)

packages/cli/src/ui/contexts/SessionContext.tsx

@@ -0,0 +1,38 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import React, { createContext, useContext, useState, useMemo } from 'react';
+
+interface SessionContextType {
+  startTime: Date;
+}
+
+const SessionContext = createContext<SessionContextType | null>(null);
+
+export const SessionProvider: React.FC<{ children: React.ReactNode }> = ({
+  children,
+}) => {
+  const [startTime] = useState(new Date());
+
+  const value = useMemo(
+    () => ({
+      startTime,
+    }),
+    [startTime],
+  );
+
+  return (
+    <SessionContext.Provider value={value}>{children}</SessionContext.Provider>
+  );
+};
+
+export const useSession = () => {
+  const context = useContext(SessionContext);
+  if (!context) {
+    throw new Error('useSession must be used within a SessionProvider');
+  }
+  return context;
+};