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

Providers/GroqProvider.cs

@@ -0,0 +1,61 @@
+using System.Net.Http.Headers;
+
+namespace AnchorCli.Providers;
+
+/// <summary>
+/// Token extractor for Groq responses.
+/// </summary>
+internal sealed class GroqTokenExtractor : ITokenExtractor
+{
+    public string ProviderName => "Groq";
+
+    public (int inputTokens, int outputTokens)? ExtractTokens(HttpResponseHeaders headers, string? responseBody)
+    {
+        // Groq provides x-groq-tokens header (format: "n;<prompt_tokens>,n;<completion_tokens>")
+        if (headers.TryGetValues("x-groq-tokens", out var values))
+        {
+            var tokenStr = values.FirstOrDefault();
+            if (!string.IsNullOrEmpty(tokenStr))
+            {
+                // Parse format: "n;123,n;45" where first is prompt, second is completion
+                var parts = tokenStr.Split(',');
+                if (parts.Length >= 2)
+                {
+                    var inputPart = parts[0].Trim();
+                    var outputPart = parts[1].Trim();
+
+                    // Extract numbers after "n;"
+                    if (inputPart.StartsWith("n;") && outputPart.StartsWith("n;"))
+                    {
+                        if (int.TryParse(inputPart[2..], out var input) &&
+                            int.TryParse(outputPart[2..], out var output))
+                        {
+                            return (input, output);
+                        }
+                    }
+                }
+            }
+        }
+
+        // Fallback: try parsing from response body
+        if (!string.IsNullOrEmpty(responseBody))
+        {
+            // TODO: Parse usage from JSON body if headers aren't available
+        }
+
+        return null;
+    }
+
+    public int? ExtractLatency(HttpResponseHeaders headers)
+    {
+        if (headers.TryGetValues("x-groq-response-time", out var values))
+        {
+            if (int.TryParse(values.FirstOrDefault(), out var latency))
+            {
+                return latency;
+            }
+        }
+
+        return null;
+    }
+}