add context window monitor

Writes the current context window to logs/current_context_window (uses a symlink to a session ID).

This PR was unfortunately generated by a different LLM and did a ton of superficial reformating, it's actually a fairly small and benign change, but I don't want to roll back everything. Hope that's ok.

crates/g3-computer-control/examples/test_permission_prompt.rs

@@ -3,10 +3,13 @@ use g3_computer_control::create_controller;
 #[tokio::main]
 async fn main() {
     println!("Testing screenshot with permission prompt...");
-    
+
     let controller = create_controller().expect("Failed to create controller");
-    
-    match controller.take_screenshot("/tmp/test_with_prompt.png", None, None).await {
+
+    match controller
+        .take_screenshot("/tmp/test_with_prompt.png", None, None)
+        .await
+    {
         Ok(_) => {
             println!("\n✅ Screenshot saved to /tmp/test_with_prompt.png");
             println!("Opening screenshot...");