ci: promote dev → main (prod), not a separate prod branch (#3)

Aligns the promotion target with the locked migration architecture
(main = PROD, dev = integration). Plain ref push is fast-forward-only;
branch protection on main rejects non-FF pushes, so a diverged main
fails loudly instead of force-moving prod.

.github/workflows/promote_dev_to_prod.yml

@@ -1,4 +1,4 @@
-name: Promote dev to prod
+name: Promote dev to main (prod)
 
 permissions:
   contents: write
@@ -9,7 +9,7 @@ on:
   workflow_dispatch:
 
 concurrency:
-  group: promote-dev-to-prod
+  group: promote-dev-to-main
   cancel-in-progress: false
 
 jobs:
@@ -20,6 +20,8 @@ jobs:
         with:
           ref: dev
           fetch-depth: 0
-      - name: Fast-forward prod to dev
+      - name: Fast-forward main (PROD) to dev
+        # main is the production branch; a plain ref push is fast-forward-only
+        # (branch protection rejects non-FF), so a diverged main fails loudly.
         run: |
-          git push origin HEAD:refs/heads/prod
+          git push origin HEAD:refs/heads/main