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/NerEntityBuilder.cs

@@ -0,0 +1,33 @@
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Tests.TestSupport;
+
+public static class NerEntityBuilder
+{
+    public static IReadOnlyList<PiiEntity> BuildFromScenario(PromptScenario scenario)
+    {
+        var entities = new List<PiiEntity>();
+
+        foreach (var (type, value) in scenario.ExpectedTypes.Zip(scenario.MustNotContainInSanitized))
+        {
+            if (type != PiiEntityType.Person)
+            {
+                continue;
+            }
+
+            var searchStart = 0;
+            while ((searchStart = scenario.Prompt.IndexOf(value, searchStart, StringComparison.Ordinal)) >= 0)
+            {
+                entities.Add(new PiiEntity(
+                    PiiEntityType.Person,
+                    value,
+                    searchStart,
+                    value.Length,
+                    PiiDetectionSource.Ner));
+                searchStart += value.Length;
+            }
+        }
+
+        return entities;
+    }
+}