fix: skip SSH deploy gracefully when deployment secrets are not set

[Copilot]

The deploy-production job was failing after ~7s with Error: missing server host because appleboy/ssh-action was invoked unconditionally with unset secrets.

Changes

• .github/workflows/deploy.yml: Added a Check deployment credentials step to deploy-production that gates the SSH deploy on the presence of DEPLOY_HOST, DEPLOY_USER, and DEPLOY_SSH_KEY — matching the existing pattern in push-docker-images for Docker credentials.

- name: Check deployment credentials
  id: creds
  run: |
    if [ -n "$DEPLOY_HOST" ] && [ -n "$DEPLOY_USER" ] && [ -n "$DEPLOY_SSH_KEY" ]; then
      echo "available=true" >> "$GITHUB_OUTPUT"
    else
      echo "available=false" >> "$GITHUB_OUTPUT"
      echo "::warning::DEPLOY_HOST, DEPLOY_USER, or DEPLOY_SSH_KEY secret is not set — skipping SSH deploy."
    fi

- name: SSH deploy
  if: steps.creds.outputs.available == 'true'
  uses: appleboy/ssh-action@v1.0.0
  ...

When secrets are absent the job emits a warning and succeeds; when configured, behaviour is unchanged.

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