ci: enable GitHub Actions for 0.4.x branch

Add 0.4.x to the list of branches that trigger CI runs in the GitHub
Actions workflow. This ensures that pushes and pull requests targeting
the stable release branch receive the same automated testing as master.

Assisted-by: Claude Code

Author: twaugh

.github/workflows/ci.yml

@@ -2,9 +2,9 @@ name: CI
 
 on:
   push:
-    branches: [ master, main ]
+    branches: [ master, main, 0.4.x ]
   pull_request:
-    branches: [ master, main ]
+    branches: [ master, main, 0.4.x ]
 
 jobs:
   test: