ci: serialize deploy jobs with a shared concurrency group

Add a job-level concurrency group (declarative-deploy) shared across
the staging and production deploy jobs so they can't push to the
GitOps repo at the same time. Builds still run in parallel.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: levivannoort

.github/workflows/production.yml

@@ -56,6 +56,9 @@ jobs:
   deploy:
     needs: build
     runs-on: ubuntu-latest
+    concurrency:
+      group: declarative-deploy
+      cancel-in-progress: false
     steps:
       - name: Get token for ${{ env.DECLARATIVE_REPOSITORY }}
         id: app-token

.github/workflows/staging.yml

@@ -57,6 +57,9 @@ jobs:
   deploy:
     needs: build
     runs-on: ubuntu-latest
+    concurrency:
+      group: declarative-deploy
+      cancel-in-progress: false
     steps:
       - name: Get token for ${{ env.DECLARATIVE_REPOSITORY }}
         id: app-token