add branding (#51)

<img width="110" height="106" alt="image"
src="https://github.com/user-attachments/assets/900b1179-18c4-4a74-a3ab-d87165ec877a"
/>

Signed-off-by: Colin Douglas <colin@chainguard.dev>

Author: cmdpdx

action.yaml

@@ -6,6 +6,10 @@ description: |
   This action sets up the Chainguard chainctl CLI and authenticates
   it against the target environment.
 
+branding:
+  icon: "terminal"
+  color: "purple"
+
 inputs:
   environment:
     description: |