ci: setup github sync

.gitea/notice.md

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+## Where this project lives
+
+The real home for this project is a self-hosted Gitea instance: **[git.scient.ing/education/coursebank](https://git.scient.ing/education/coursebank)**, where the code, issues, and history live on infrastructure I control.
+GitHub only holds a pointer, so open issues, send pull requests, and clone from the link above.
+I moved because I would rather own where my code lives than rent it under terms a platform can rewrite by announcement, and self-hosting puts backups, uptime, and data location back in my hands.
+Old GitHub links still resolve here, so nothing breaks.

.gitea/workflows/sync-readme.yml

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+name: Sync README to GitHub
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'README.md'
+      - '.gitea/workflows/sync-readme.yml'
+      - '.gitea/notice.md'
+
+jobs:
+  sync:
+    runs-on: github-readme-sync
+    steps:
+      - name: Check out the Gitea repo
+        uses: actions/checkout@v4
+
+      - name: Build the combined README
+        run: |
+          cat README.md > combined.md
+          printf '\n\n' >> combined.md
+          cat .gitea/notice.md >> combined.md
+
+      - name: Push to the GitHub mirror
+        env:
+          GH_TOKEN: ${{ secrets.GH_MIRROR_TOKEN }}
+        run: |
+          git clone --depth 1 \
+            "https://x-access-token:${GH_TOKEN}@github.com/scienting/coursebank.git" ghmirror
+          cp combined.md ghmirror/README.md
+          cd ghmirror
+          git config user.name "Alex Maldonado"
+          git config user.email "alexm@scient.ing"
+          git add README.md
+          git diff --cached --quiet || git commit -m "Sync README from Gitea"
+          git push