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-ensembles/src/tests.rs

@@ -283,8 +283,7 @@ mod tests {
         assert!(json.contains("Completed"));
 
         // Deserialize back
-        let deserialized: FlockStatus =
-            serde_json::from_str(&json).expect("Failed to deserialize");
+        let deserialized: FlockStatus = serde_json::from_str(&json).expect("Failed to deserialize");
         assert_eq!(deserialized.session_id, "test-session");
         assert_eq!(deserialized.segments.len(), 1);
         assert_eq!(deserialized.total_tokens, 1000);