ci: nightly promote of main to prod for LangGraph deploys (#1285)

Adds a scheduled workflow that force-pushes main onto a prod branch at
08:00 UTC (midnight PST / 1am PDT). The LangGraph deployment will track
prod instead of main, so routine merges no longer trigger redeploys
that cancel in-flight 30-minute coding runs. workflow_dispatch is
retained for hotfixes that need to ship immediately.

.github/workflows/promote_main_to_prod.yml

@@ -0,0 +1,25 @@
+name: Promote main to prod
+
+permissions:
+  contents: write
+
+on:
+  schedule:
+    - cron: "0 8 * * *"
+  workflow_dispatch:
+
+concurrency:
+  group: promote-main-to-prod
+  cancel-in-progress: false
+
+jobs:
+  promote:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          ref: main
+          fetch-depth: 0
+      - name: Force-push main to prod
+        run: |
+          git push --force origin HEAD:refs/heads/prod