Add PII redaction POC for secure LLM prompting.

Implements detect-redact-sanitize pipeline with regex, domain rules, and ONNX NER before the LLM boundary, plus NUnit tests and Xenovex push documentation.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/TestSupport.Shared/RealNerModelPaths.cs

@@ -0,0 +1,24 @@
+namespace PiiRedaction.Tests.Shared;
+
+public static class RealNerModelPaths
+{
+    public const string ModelMissingMessage =
+        "ONNX model not found. Run scripts/download-ner-model.ps1 from the repository root.";
+
+    public static string ResolveRepoModelPath()
+    {
+        var directory = new DirectoryInfo(AppContext.BaseDirectory);
+        while (directory is not null)
+        {
+            var candidate = Path.Combine(directory.FullName, "models", "ner-model.onnx");
+            if (File.Exists(candidate))
+            {
+                return candidate;
+            }
+
+            directory = directory.Parent;
+        }
+
+        return Path.Combine(Environment.CurrentDirectory, "models", "ner-model.onnx");
+    }
+}