Fix tool invocation, add NUnit constraints, wire attributes

Ngentic.slnx

@@ -1,9 +1,9 @@
 <Solution>
-  <Folder Name="/samples/">
-    <Project Path="samples/Ngentic.Samples/Ngentic.Samples.csproj" />
-  </Folder>
   <Folder Name="/src/">
     <Project Path="src/Ngentic.Core/Ngentic.Core.csproj" />
     <Project Path="src/Ngentic.NUnit/Ngentic.NUnit.csproj" />
   </Folder>
+  <Folder Name="/tests/">
+    <Project Path="tests/Ngentic.Tests/Ngentic.Tests.csproj" />
+  </Folder>
 </Solution>

src/Ngentic.Core/AgentRun.cs

@@ -8,4 +8,18 @@ public sealed class AgentRun
     public required TimeSpan Duration { get; init; }
     public required int Turns { get; init; }
     public required bool HitTurnLimit { get; init; }
+
+    /// <summary>
+    /// Total cost reported by the agent driver, in USD. Populated by the
+    /// Claude CLI driver from the final stream-json `result` event; null
+    /// when the driver does not surface cost.
+    /// </summary>
+    public double? CostUsd { get; init; }
+
+    /// <summary>
+    /// Free-form payload for driver-specific information that does not earn
+    /// a top-level field. Populated by the Claude CLI driver with stderr,
+    /// exit code, and the raw stream-json transcript for debugging.
+    /// </summary>
+    public IReadOnlyDictionary<string, object?>? Metadata { get; init; }
 }

src/Ngentic.Core/Args.cs

@@ -0,0 +1,20 @@
+namespace Ngentic;
+
+/// <summary>
+/// Lightweight builder for the <c>Dictionary&lt;string, object?&gt;</c> argument
+/// bag that MCP client APIs typically take. Lets tests write
+/// <c>Args.Of(("slot", slotA), ("script", "..."))</c> instead of a full
+/// dictionary initializer.
+/// </summary>
+public static class Args
+{
+    public static Dictionary<string, object?> Of(params (string name, object? value)[] pairs)
+    {
+        Dictionary<string, object?> result = new(pairs.Length);
+        foreach ((string name, object? value) in pairs)
+        {
+            result[name] = value;
+        }
+        return result;
+    }
+}