chore: add Serena AI assistant entries to .gitignore

Add .serena/ directory and *.serena-memory files to prevent
tracking of AI assistant configuration and memory files.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -44,3 +44,7 @@ mongo-volume-backup-*.tar.gz
 
 # backups directory for mongo-volume tarballs
 /backups/
+
+# Serena AI assistant
+.serena/
+*.serena-memory