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/PromptScenario.cs

@@ -0,0 +1,10 @@
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Tests.TestSupport;
+
+public sealed record PromptScenario(
+    string Name,
+    string Prompt,
+    string ExpectedSanitized,
+    PiiEntityType[] ExpectedTypes,
+    string[] MustNotContainInSanitized);