github: automatically close PRs from the master branch.

This GitHub Actions workflow detects pull requests originating from the
`master` branch and automatically closes them. It also leaves a comment
explaining the reason and guiding contributors to create a feature
branch instead.

Signed-off-by: Nikolay Martyanov <[email protected]>

.github/workflows/close-master-pr.yml

@@ -0,0 +1,41 @@
+# Copyright (c) 2025, Zededa, Inc.
+# SPDX-License-Identifier: Apache-2.0
+---
+name: Close PRs from master
+
+on:  # yamllint disable-line rule:truthy
+  pull_request:
+    types: [opened, reopened]
+
+jobs:
+  close-master-pr:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Close PR if from master
+        if: github.event.pull_request.head.ref == 'master' || github.event.pull_request.head.ref == 'main'
+        run: |
+          # 1) Close the PR via GitHub API
+          curl -s -X PATCH \
+            -H "Authorization: Bearer ${{ secrets.GITHUB_TOKEN }}" \
+            -H "Accept: application/vnd.github.v3+json" \
+            -d '{"state":"closed"}' \
+            "https://api.github.com/repos/${{ github.repository }}/pulls/${{ github.event.pull_request.number }}"
+
+          # 2) Add a comment explaining why
+          COMMENT_PAYLOAD=$(cat <<EOF
+          {
+            "body": "Hi! We automatically close PRs from the \`master\` branch.\n\n\
+            Please create a dedicated feature branch. For example:\n\n\
+            1. In your fork, create a new branch named \`feature/my-change\`.\n\
+            2. Commit and push your changes to that branch.\n\
+            3. Open a new pull request from your \`feature/my-change\` branch.\n\n\
+            Thanks!"
+          }
+          EOF
+          )
+
+          curl -s -X POST \
+            -H "Authorization: Bearer ${{ secrets.GITHUB_TOKEN }}" \
+            -H "Accept: application/vnd.github.v3+json" \
+            -d "$COMMENT_PAYLOAD" \
+            "https://api.github.com/repos/${{ github.repository }}/issues/${{ github.event.pull_request.number }}/comments"