audit, advisories and doctors/setup work

src/AdvisoryAI/StellaOps.AdvisoryAI/Explanation/NullExplanationInferenceClient.cs

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+// <copyright file="NullExplanationInferenceClient.cs" company="StellaOps">
+// Copyright (c) StellaOps. Licensed under the AGPL-3.0-or-later.
+// </copyright>
+using System.Security.Cryptography;
+using System.Text;
+
+namespace StellaOps.AdvisoryAI.Explanation;
+
+public sealed class NullExplanationInferenceClient : IExplanationInferenceClient
+{
+    public Task<ExplanationInferenceResult> GenerateAsync(
+        ExplanationPrompt prompt,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(prompt);
+
+        var promptHash = ComputeHash(prompt.Content ?? string.Empty);
+        var content = $"Placeholder explanation (no model). prompt_hash=sha256:{promptHash}";
+
+        return Task.FromResult(new ExplanationInferenceResult
+        {
+            Content = content,
+            Confidence = 0.0,
+            ModelId = "stub-explainer:v0"
+        });
+    }
+
+    private static string ComputeHash(string content)
+    {
+        var bytes = SHA256.HashData(Encoding.UTF8.GetBytes(content));
+        return Convert.ToHexStringLower(bytes);
+    }
+}