add acceptance test

.github/workflows/acceptance.yml

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+name: acceptance
+
+on:
+  pull_request:
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  acceptance:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      # if on pull request
+      - uses: ./
+        if: github.event_name == 'pull_request'
+        with:
+            file: demo/acceptance.md
+            vars: |
+              sha: ${{ github.sha }}
+              environment: production
+              app: cool-app

demo/acceptance.md

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+# Acceptance Test
+
+This comment is from the acceptance test. If you are seeing this, it means the acceptance test has passed! 🎉
+
+**{{ app }}** has been successfully deployed to **{{ environment }}**
+
+- SHA: `{{ sha }}`