Add llm.txt to .gitignore to keep LLM knowledge base local only

The llm.txt file contains comprehensive project documentation and
should remain local as it may contain sensitive information about
the project structure and implementation details.

.gitignore

@@ -65,6 +65,9 @@ Thumbs.db
 deployment_info.txt
 CACHE_MANAGEMENT.md
 
+# LLM Knowledge Base (keep local only)
+llm.txt
+
 # Jupyter
 .ipynb_checkpoints/