feat: Implement proper branch-based auto-promotion strategy

🚀 **New Branching Strategy:**
- develop → triggers dev deployment → auto-promotes to staging branch
- staging → triggers staging deployment → manual approval → promotes to main branch
- main → triggers production deployment

📝 **Workflow Changes:**
- deploy-dev.yml: Now triggers on develop branch
- deploy-staging.yml: Now triggers on staging branch push
- deploy-prod.yml: Now triggers on main branch push
- auto-promote.yml: Tests dev → merges develop to staging branch
- promote-to-production.yml: Tests staging → requires approval → merges staging to main
- build-image.yml: Now builds on all branches (main, develop, staging)

🎯 **Auto-Promotion Flow:**
1. Push to develop → Deploy to dev → Test → Auto-merge to staging
2. Staging deployment → Test → Manual approval → Auto-merge to main
3. Main deployment → Production live!

This provides proper separation between environments with appropriate gates.

.github/workflows/deploy-prod.yml

@@ -1,6 +1,8 @@
 name: Deploy to Production
 
 on:
+  push:
+    branches: [ main ]
   workflow_dispatch:
     inputs:
       image_tag: