fix(ci): rotate staging SSH key and add connection debug step

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/deploy-staging.yml

@@ -87,6 +87,13 @@ jobs:
             UserKnownHostsFile /dev/null
           EOF
 
+      - name: Verify SSH Key
+        run: |
+          echo "Key fingerprint:"
+          ssh-keygen -lf ~/.ssh/staging_key
+          echo "Testing SSH connection..."
+          ssh -v staging "echo SSH_CONNECTION_OK" 2>&1 | grep -E "Authentication|Offering|Accepted|debug1.*identity"
+
       - name: Deploy to Staging
         run: |
           ssh staging << 'ENDSSH'