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>

src/PiiRedaction.Core/Models/RedactionResult.cs

@@ -0,0 +1,5 @@
+namespace PiiRedaction.Core.Models;
+
+public sealed record RedactionResult(
+    string SanitizedText,
+    IReadOnlyDictionary<string, string> PlaceholderMap);