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

Providers/ITokenExtractor.cs

@@ -0,0 +1,25 @@
+using System.Net.Http.Headers;
+
+namespace AnchorCli.Providers;
+
+/// <summary>
+/// Interface for extracting token usage from provider responses.
+/// </summary>
+internal interface ITokenExtractor
+{
+    /// <summary>
+    /// Extracts token usage from response headers and/or body.
+    /// Returns (inputTokens, outputTokens) or null if unavailable.
+    /// </summary>
+    (int inputTokens, int outputTokens)? ExtractTokens(HttpResponseHeaders headers, string? responseBody);
+
+    /// <summary>
+    /// Gets the latency from response headers (in ms).
+    /// </summary>
+    int? ExtractLatency(HttpResponseHeaders headers);
+
+    /// <summary>
+    /// Gets the provider name for display purposes.
+    /// </summary>
+    string ProviderName { get; }
+}