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-core/tests/test_token_counting.rs

@@ -4,7 +4,7 @@ use g3_providers::Usage;
 #[test]
 fn test_token_accumulation() {
     let mut window = ContextWindow::new(10000);
-    
+
     // First API call: 100 prompt + 50 completion = 150 total
     let usage1 = Usage {
         prompt_tokens: 100,
@@ -22,7 +22,10 @@ fn test_token_accumulation() {
         total_tokens: 275,
     };
     window.update_usage_from_response(&usage2);
-    assert_eq!(window.used_tokens, 425, "Second call should accumulate to 425 tokens");
+    assert_eq!(
+        window.used_tokens, 425,
+        "Second call should accumulate to 425 tokens"
+    );
     assert_eq!(window.cumulative_tokens, 425, "Cumulative should be 425");
 
     // Third API call with SMALLER token count: 50 prompt + 25 completion = 75 total
@@ -32,27 +35,33 @@ fn test_token_accumulation() {
         total_tokens: 75,
     };
     window.update_usage_from_response(&usage3);
-    assert_eq!(window.used_tokens, 500, "Third call should accumulate to 500 tokens");
+    assert_eq!(
+        window.used_tokens, 500,
+        "Third call should accumulate to 500 tokens"
+    );
     assert_eq!(window.cumulative_tokens, 500, "Cumulative should be 500");
-    
+
     // Verify tokens never decrease
-    assert!(window.used_tokens >= 425, "Token count should never decrease!");
+    assert!(
+        window.used_tokens >= 425,
+        "Token count should never decrease!"
+    );
 }
 
 #[test]
 fn test_add_streaming_tokens() {
     let mut window = ContextWindow::new(10000);
-    
+
     // Add some streaming tokens
     window.add_streaming_tokens(100);
     assert_eq!(window.used_tokens, 100);
     assert_eq!(window.cumulative_tokens, 100);
-    
+
     // Add more
     window.add_streaming_tokens(50);
     assert_eq!(window.used_tokens, 150);
     assert_eq!(window.cumulative_tokens, 150);
-    
+
     // Now update from provider response
     let usage = Usage {
         prompt_tokens: 80,
@@ -60,7 +69,7 @@ fn test_add_streaming_tokens() {
         total_tokens: 120,
     };
     window.update_usage_from_response(&usage);
-    
+
     // Should ADD to existing, not replace
     assert_eq!(window.used_tokens, 270, "Should add 120 to existing 150");
     assert_eq!(window.cumulative_tokens, 270);
@@ -69,7 +78,7 @@ fn test_add_streaming_tokens() {
 #[test]
 fn test_percentage_calculation() {
     let mut window = ContextWindow::new(1000);
-    
+
     // Add tokens via provider response
     let usage = Usage {
         prompt_tokens: 150,
@@ -77,10 +86,10 @@ fn test_percentage_calculation() {
         total_tokens: 250,
     };
     window.update_usage_from_response(&usage);
-    
+
     assert_eq!(window.percentage_used(), 25.0);
     assert_eq!(window.remaining_tokens(), 750);
-    
+
     // Add more tokens
     let usage2 = Usage {
         prompt_tokens: 300,
@@ -88,7 +97,7 @@ fn test_percentage_calculation() {
         total_tokens: 500,
     };
     window.update_usage_from_response(&usage2);
-    
+
     assert_eq!(window.percentage_used(), 75.0);
     assert_eq!(window.remaining_tokens(), 250);
 }