Restrict publish workflows to pushes on main

Without a branch filter, any push on any branch touching a chart
directory packaged and published the chart to the registry.

Fixes #4

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/publish-gotenberg.yml

@@ -2,6 +2,8 @@ name: Publish Gotenberg Helm Chart
 on:
   workflow_dispatch:
   push:
+    branches:
+      - main
     paths:
       - gotenberg/**
 jobs: