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/PiiRedaction.Core.Tests/TestSupport/FakeOnnxNerModelRunner.cs

@@ -0,0 +1,19 @@
+using PiiRedaction.Core.Detection;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Tests.TestSupport;
+
+public sealed class FakeOnnxNerModelRunner : IOnnxNerModelRunner
+{
+    public bool IsModelAvailable { get; set; }
+
+    public IReadOnlyList<PiiEntity> EntitiesToReturn { get; set; } = [];
+
+    public string? LastPredictedText { get; private set; }
+
+    public IReadOnlyList<PiiEntity> PredictEntities(string text)
+    {
+        LastPredictedText = text;
+        return EntitiesToReturn;
+    }
+}