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.

examples/verify_message_id.rs

@@ -6,19 +6,19 @@ use g3_providers::{Message, MessageRole};
 fn main() {
     println!("=== Message ID Implementation Verification ===");
     println!();
-    
+
     // Create several messages to show ID generation
     println!("Creating 5 messages to demonstrate ID generation:");
     for i in 1..=5 {
         let msg = Message::new(MessageRole::User, format!("Test message {}", i));
         println!("  Message {}: id = '{}'", i, msg.id);
     }
-    
+
     println!();
     println!("ID Format: HHMMSS-XXX");
     println!("  - HHMMSS: Current time (hours, minutes, seconds)");
     println!("  - XXX: 3 random alphabetic characters (a-z, A-Z)");
-    
+
     println!();
     println!("Verifying ID is NOT serialized to JSON:");
     let msg = Message::new(MessageRole::User, "Hello World".to_string());
@@ -26,7 +26,7 @@ fn main() {
     println!("  Message ID: {}", msg.id);
     println!("  JSON output: {}", json);
     println!("  Contains 'id' field: {}", json.contains("\"id\""));
-    
+
     println!();
     println!("✅ Implementation complete!");
 }