Add sync-upstream workflow

.github/workflows/sync-upstream.yml

@@ -0,0 +1,69 @@
+name: Sync Upstream
+
+on:
+  workflow_dispatch:
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      pull-requests: write
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Configure git identity
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+
+      - name: Fetch upstream
+        run: |
+          git remote add upstream https://github.com/migueldeicaza/SwiftTerm.git
+          git fetch upstream main
+
+      - name: Check if sync needed
+        id: check
+        run: |
+          BEHIND=$(git rev-list --count HEAD..upstream/main)
+          echo "behind=$BEHIND" >> "$GITHUB_OUTPUT"
+          if [ "$BEHIND" = "0" ]; then
+            echo "main is already up to date with upstream"
+          else
+            echo "main is $BEHIND commit(s) behind upstream"
+          fi
+
+      - name: Create sync branch and merge
+        if: steps.check.outputs.behind != '0'
+        id: merge
+        run: |
+          DATE=$(date -u +%Y-%m-%d)
+          BRANCH="sync/upstream-$DATE"
+          git checkout -b "$BRANCH"
+          echo "branch=$BRANCH" >> "$GITHUB_OUTPUT"
+
+          if git merge upstream/main --no-edit; then
+            echo "conflicts=false" >> "$GITHUB_OUTPUT"
+          else
+            echo "conflicts=true" >> "$GITHUB_OUTPUT"
+            git merge --abort
+            exit 1
+          fi
+
+      - name: Push sync branch
+        if: steps.check.outputs.behind != '0' && steps.merge.outputs.conflicts == 'false'
+        run: |
+          git push --set-upstream origin "${{ steps.merge.outputs.branch }}"
+
+      - name: Open pull request
+        if: steps.check.outputs.behind != '0' && steps.merge.outputs.conflicts == 'false'
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          gh pr create \
+            --base main \
+            --head "${{ steps.merge.outputs.branch }}" \
+            --title "Sync upstream ($(date -u +%Y-%m-%d))" \
+            --body "Automated merge of \`migueldeicaza/SwiftTerm@main\` into \`main\`. Review the diff and merge if our customizations still compile and behave correctly."