fix: detect critical stacks only for existing stacks, not new ones

New stacks don't have compose files on the remote server yet, causing
"Compose file not found" warnings during critical stack detection.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: owine

.github/workflows/deploy.yml

@@ -596,10 +596,10 @@ jobs:
 
       - name: Detect critical stacks
         id: detect-critical
-        if: steps.backup.outputs.deployment_needed == 'true' && inputs.auto-detect-critical == true
+        if: steps.backup.outputs.deployment_needed == 'true' && inputs.auto-detect-critical == true && steps.detect-changes.outputs.has_existing_stacks == 'true'
         run: |
           ./.compose-workflow/scripts/deployment/detect-critical-stacks.sh \
-            --stacks "${{ join(fromJSON(steps.discover-stacks.outputs.stacks), ' ') }}" \
+            --stacks "${{ join(fromJSON(steps.detect-changes.outputs.existing_stacks), ' ') }}" \
             --ssh-user "${{ secrets.SSH_USER }}" \
             --ssh-host "${{ secrets.SSH_HOST }}"