docs(llms.txt): add Last-checked date

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Lukas Geiger

llms.txt

@@ -30,3 +30,5 @@ Core concepts: Chain (JSON definition with ordered links), Link (one agent with
 ## Optional
 
 - [GitHub Issues](https://github.com/ellmos-ai/MarbleRun/issues): Bug reports and feature requests
+
+## Last-checked: 2026-06-10