feat: explicit restriction to main branch and closed PRs

Author: cmeesters

.github/workflows/post_to_mastodon.yml

@@ -2,9 +2,14 @@ name: Post to Mastodon on PR Merge
 
 on:
   pull_request:
+    branches:
+      - main
     types:
       - closed
 
+permissions:
+  pull-request: read
+
 jobs:
   post_to_mastodon:
     if: github.event.pull_request.merged == true