feat: saving token data to sessions

ChatSession.cs

@@ -8,6 +8,11 @@ internal sealed class ChatSession
     private readonly IChatClient _agent;
     public ContextCompactor Compactor { get; }
     public List<ChatMessage> History { get; }
+    
+    // Token tracking state persisted across sessions
+    public long SessionInputTokens { get; set; }
+    public long SessionOutputTokens { get; set; }
+    public int RequestCount { get; set; }
 
     public ChatSession(IChatClient innerClient)
     {
@@ -84,6 +89,18 @@ internal sealed class ChatSession
         };
         
         var json = JsonSerializer.Serialize(messagesToSave, AppJsonContext.Default.ListChatMessage);
+        
+        // Save token stats to a separate metadata file
+        var metadataPath = Path.ChangeExtension(filePath, ".metadata.json");
+        var metadata = new TokenMetadata
+        {
+            SessionInputTokens = SessionInputTokens,
+            SessionOutputTokens = SessionOutputTokens,
+            RequestCount = RequestCount
+        };
+        var metadataJson = JsonSerializer.Serialize(metadata, AppJsonContext.Default.TokenMetadata);
+        await File.WriteAllTextAsync(metadataPath, metadataJson, cancellationToken);
+        
         await File.WriteAllTextAsync(filePath, json, cancellationToken);
     }
 
@@ -104,5 +121,33 @@ internal sealed class ChatSession
         History.Clear();
         History.Add(systemMessage);
         History.AddRange(messages);
+        
+        // Load token stats from metadata file if it exists
+        var metadataPath = Path.ChangeExtension(filePath, ".metadata.json");
+        if (File.Exists(metadataPath))
+        {
+            try
+            {
+                var metadataJson = await File.ReadAllTextAsync(metadataPath, cancellationToken);
+                var metadata = JsonSerializer.Deserialize<TokenMetadata>(metadataJson, AppJsonContext.Default.TokenMetadata);
+                if (metadata != null)
+                {
+                    SessionInputTokens = metadata.SessionInputTokens;
+                    SessionOutputTokens = metadata.SessionOutputTokens;
+                    RequestCount = metadata.RequestCount;
+                }
+            }
+            catch { /* Ignore metadata load errors */ }
+        }
     }
 }
+
+/// <summary>
+/// Token tracking metadata serialized with the session.
+/// </summary>
+internal sealed class TokenMetadata
+{
+    public long SessionInputTokens { get; set; }
+    public long SessionOutputTokens { get; set; }
+    public int RequestCount { get; set; }
+}