Add task_categories and remove arxiv from metadata

This PR adds the `task_categories` metadata (robotics) and removes the `arxiv` field from YAML, which should only appear in the markdown content section as per the Hugging Face guidelines.

README.md

@@ -1,13 +1,14 @@
 ---
-license: apache-2.0
 language:
 - en
+license: apache-2.0
 tags:
 - vla
 - robotics
 - embodied-ai
 - vision-language-action
-arxiv: 2608.15875
+task_categories:
+- robotics
 ---
 
 <div align="center" style="font-family: charter;">
@@ -58,4 +59,4 @@ scenarios.
   primaryClass={cs.RO},
   url={https://arxiv.org/abs/2608.15875},
 }
-```
+```