Add decision-model tag

Hi Perplexity team,

This PR adds the `decision-model` tag to the model card metadata, so pplx-decider-v1.1-27b shows up next to the other decision models on the Hub: https://huggingface.co/models?other=decision-model

<img src="https://cdn-uploads.huggingface.co/production/uploads/noauth/gKtmJfAU6e4uyiBQRFjU7.png" alt="decision-model tag" width="500">

It's a one-line metadata change, nothing else in the card is touched.

README.md

@@ -9,6 +9,7 @@ tags:
   - multimodal
   - custom-code
   - decider
+  - decision-model
 ---
 
 # pplx-decider-v1.1-27b