Add workflow to auto-propagate main changes to deployment branches

Merges main into modal and hetzner on every push. Opens a PR
instead when there are merge conflicts.

Closes #169

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: chriscarrollsmith

.github/workflows/propagate.yml

@@ -0,0 +1,66 @@
+name: propagate
+
+on:
+  push:
+    branches: [main]
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  propagate:
+    strategy:
+      matrix:
+        branch: [modal, hetzner]
+    runs-on: ubuntu-latest
+    steps:
+      - name: Generate token
+        id: create_token
+        uses: tibdex/github-app-token@v2
+        with:
+          app_id: ${{ secrets.APP_ID }}
+          private_key: ${{ secrets.PRIVATE_KEY }}
+
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          token: ${{ steps.create_token.outputs.token }}
+
+      - name: Attempt merge and push, or open PR on conflict
+        env:
+          GH_TOKEN: ${{ steps.create_token.outputs.token }}
+          BRANCH: ${{ matrix.branch }}
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+
+          git checkout "$BRANCH"
+
+          if git merge origin/main --no-edit; then
+            echo "Merge succeeded, pushing to $BRANCH"
+            git push origin "$BRANCH"
+          else
+            echo "Merge conflict detected, opening PR"
+            git merge --abort
+
+            # Create a temporary branch for the PR
+            TEMP_BRANCH="auto-merge/main-to-${BRANCH}-$(date +%Y%m%d%H%M%S)"
+            git checkout -b "$TEMP_BRANCH" origin/main
+
+            git push origin "$TEMP_BRANCH"
+
+            gh pr create \
+              --base "$BRANCH" \
+              --head "$TEMP_BRANCH" \
+              --title "Propagate main to ${BRANCH}" \
+              --body "$(cat <<'PREOF'
+          ## Summary
+          - Auto-generated PR to propagate changes from `main` to the deployment branch
+          - Opened because an automatic merge had conflicts that need manual resolution
+
+          🤖 Generated by the propagate workflow
+          PREOF
+              )"
+          fi