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>

.gitignore

@@ -0,0 +1,23 @@
+## .NET
+bin/
+obj/
+*.user
+*.suo
+.vs/
+
+## ONNX models (place downloaded models here locally)
+models/*.onnx
+models/vocab.txt
+models/ner-labels.txt
+models/*.json
+!models/.gitkeep
+
+## IDE
+.idea/
+*.swp
+
+## Local experiments (not part of the solution)
+scratch/
+
+## Test artifacts
+**/nunit_random_seed.tmp