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.ConsoleApp/appsettings.json

@@ -0,0 +1,5 @@
+{
+  "PiiRedaction": {
+    "OnnxModelPath": "models/ner-model.onnx"
+  }
+}