CI: weekly PR merge to main from desktop and laptop

.github/workflows/sync-main.yml

@@ -0,0 +1,58 @@
+name: Sync branches into main
+
+on:
+  schedule:
+    - cron: "0 14 * * 1"
+
+    - cron: "0 14 * * 3"
+
+  workflow_dispatch:
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Determine source branch
+        id: branch
+        run: |
+          DOW=$(date -u +%u)
+
+          if [ "$DOW" = "1" ]; then
+            echo "source=laptop" >> "$GITHUB_OUTPUT"
+          elif [ "$DOW" = "3" ]; then
+            echo "source=desktop" >> "$GITHUB_OUTPUT"
+          else
+            echo "Not a scheduled sync day"
+            exit 1
+          fi
+
+      - name: Create pull request
+        id: cpr
+        uses: peter-evans/create-pull-request@v7
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          base: main
+          branch: ${{ steps.branch.outputs.source }}
+          title: "Weekly sync: ${{ steps.branch.outputs.source }} → main"
+          body: |
+            This pull request was created automatically by GitHub Actions.
+
+            If the branch can be merged cleanly, auto-merge will merge it automatically.
+
+            If there are merge conflicts, this PR will remain open until they are resolved.
+          draft: false
+
+      - name: Enable auto-merge
+        if: steps.cpr.outputs.pull-request-number
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          gh pr merge \
+            ${{ steps.cpr.outputs.pull-request-number }} \
+            --auto \
+            --merge