chore: add Windows nul device to .gitignore

Windows shell redirections and tooling can create a literal file named 'nul' (a reserved device name) that cannot be deleted via normal rm operations. Adding it to .gitignore prevents this artifact from polluting improvement branches on Windows clones.

Author: cowork-bot

.gitignore

@@ -66,6 +66,7 @@ ENV/
 # OS
 .DS_Store
 Thumbs.db
+nul
 
 # Project specific
 research/