Implement browser session inactivity timeout and cleanup

- Updated `.env.example` to include `BROWSER_INACTIVITY_TIMEOUT` for auto-cleanup of inactive sessions.
- Enhanced `cli.py` to load the new inactivity timeout configuration into environment variables.
- Added background thread functionality in `browser_tool.py` to periodically clean up inactive browser sessions based on the configured timeout.
- Improved session management by tracking last activity timestamps and ensuring cleanup occurs when sessions exceed inactivity limits.

cli.py

@@ -67,6 +67,9 @@ def load_cli_config() -> Dict[str, Any]:
             "singularity_image": "docker://python:3.11",
             "modal_image": "python:3.11",
         },
+        "browser": {
+            "inactivity_timeout": 120,  # Auto-cleanup inactive browser sessions after 2 min
+        },
         "agent": {
             "max_turns": 20,
             "verbose": False,
@@ -138,6 +141,16 @@ def load_cli_config() -> Dict[str, Any]:
         if config_key in terminal_config:
             os.environ[env_var] = str(terminal_config[config_key])
     
+    # Apply browser config to environment variables
+    browser_config = defaults.get("browser", {})
+    browser_env_mappings = {
+        "inactivity_timeout": "BROWSER_INACTIVITY_TIMEOUT",
+    }
+    
+    for config_key, env_var in browser_env_mappings.items():
+        if config_key in browser_config:
+            os.environ[env_var] = str(browser_config[config_key])
+    
     return defaults
 
 # Load configuration at module startup