Add optional API URL configurations

Added optional API URL configurations for BrowserBase and Stagehand.

Author: pirate

.env.example

@@ -12,3 +12,7 @@ OPENAI_API_KEY=your_openai_key                      # OpenAI
 # GROQ_API_KEY=your_groq_key                        # Groq
 # CEREBRAS_API_KEY=your_cerebras_key                # Cerebras
 # DEEPSEEK_API_KEY=your_deepseek_key                # DeepSeek
+
+# Optional:
+# BROWSERBASE_API_URL=https://api.browserbase.com/v1
+# STAGEHAND_API_URL=https://api.stagehand.browserbase.com/v1