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/Configuration/PiiRedactionOptions.cs

@@ -0,0 +1,8 @@
+namespace PiiRedaction.Core.Configuration;
+
+public sealed class PiiRedactionOptions
+{
+    public const string SectionName = "PiiRedaction";
+
+    public string OnnxModelPath { get; set; } = "models/ner-model.onnx";
+}