feat: Introduce a pluggable LLM provider system with token extraction, pricing, and updated setup configuration.

Providers/OpenRouterTokenExtractor.cs

@@ -0,0 +1,42 @@
+using System.Net.Http.Headers;
+
+namespace AnchorCli.Providers;
+
+/// <summary>
+/// Token extractor for OpenRouter responses.
+/// </summary>
+internal sealed class OpenRouterTokenExtractor : ITokenExtractor
+{
+    public string ProviderName => "OpenRouter";
+
+    public (int inputTokens, int outputTokens)? ExtractTokens(HttpResponseHeaders headers, string? responseBody)
+    {
+        // OpenRouter provides x-total-tokens header
+        if (headers.TryGetValues("x-total-tokens", out var values))
+        {
+            // Note: OpenRouter only provides total tokens, not split
+            // We'll estimate split based on typical ratios if needed
+            if (long.TryParse(values.FirstOrDefault(), out var total))
+            {
+                // For now, return total as output (placeholder until we have better splitting)
+                // In practice, you'd need to track input separately from the request
+                return (0, (int)total);
+            }
+        }
+
+        return null;
+    }
+
+    public int? ExtractLatency(HttpResponseHeaders headers)
+    {
+        if (headers.TryGetValues("x-response-timing", out var values))
+        {
+            if (int.TryParse(values.FirstOrDefault(), out var latency))
+            {
+                return latency;
+            }
+        }
+
+        return null;
+    }
+}