ci: make the contribution reminder generic

.github/workflows/contribution-reminder.yml

@@ -0,0 +1,31 @@
+name: Contribution reminder
+
+# Asks LeanCode organization members to confirm that their PR is meant to be
+# public, since everything merged here is publicly visible.
+#
+# pull_request_target is safe here: the job never checks out or executes PR
+# code. It only reads the event payload and posts a comment.
+
+on:
+  pull_request_target:
+    types: [opened]
+
+permissions:
+  pull-requests: write
+
+jobs:
+  remind:
+    if: contains(fromJSON('["MEMBER", "OWNER"]'), github.event.pull_request.author_association)
+    runs-on: ubuntu-latest
+    steps:
+      - name: Comment with a public-visibility reminder
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          AUTHOR: ${{ github.event.pull_request.user.login }}
+        run: |
+          gh pr comment "$PR_URL" --body "Hi @${AUTHOR}! :wave: A quick check before this ships: **is this PR meant to be public?**
+
+          Everything merged here is publicly visible. If any of it should stay inside LeanCode, it doesn't belong in this repository.
+
+          If public is what you intended, just ignore this comment. :slightly_smiling_face:"