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-console/src/models/message.rs

@@ -1,5 +1,5 @@
-use serde::{Deserialize, Serialize};
 use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ChatMessage {