feat: Add staging environment configuration with protection rules, approvals, and environment variables

- Add staging.yml with environment name/URL and protection rules configuration
- Add required_reviewers (staging-team-lead/devops-engineer) and prevent_force_pushes setting
- Add required_status_checks with strict mode for build-and-push/test-suite contexts
- Add environment_variables for API_BASE_URL/DATABASE_URL/REDIS_URL/LOG_LEVEL/ENVIRONMENT
- Add secrets references for STAGING_DB_PASSWORD/

.github/environments/staging.yml

@@ -0,0 +1,36 @@
+# Staging Environment Configuration
+name: staging
+url: https://staging.telecom-platform.com
+
+# Environment protection rules
+protection_rules:
+  # Require approval from staging team
+  required_reviewers:
+    - staging-team-lead
+    - devops-engineer
+  
+  # Prevent force pushes
+  prevent_force_pushes: true
+  
+  # Require status checks
+  required_status_checks:
+    strict: true
+    contexts:
+      - build-and-push
+      - test-suite
+
+# Environment variables
+environment_variables:
+  API_BASE_URL: https://api.staging.telecom-platform.com
+  DATABASE_URL: postgresql://staging-user:[email protected]:5432/telecom_staging
+  REDIS_URL: redis://staging-redis.internal:6379
+  LOG_LEVEL: info
+  ENVIRONMENT: staging
+
+# Secrets (configured in GitHub repository settings)
+# These are referenced but not stored in the file
+secrets:
+  - STAGING_DB_PASSWORD
+  - STAGING_API_KEY
+  - STAGING_JWT_SECRET
+  - KUBECONFIG_STAGING