Add DRUID image with Singularity cluster deployment

[byee4]

Summary

• add the validated DRUID complete-analysis image and its automated GHCR publication
• fix artifact collection by running the mounted smoke test as the GitHub Actions runner user
• document a Singularity-first, Dockerless cluster workflow using the pinned DRUID image

Validation

• GitHub Actions run 34260602568 passed the synthetic DRUID workflow, image publish, and validation-artifact upload.
• git diff --check and workflow YAML parsing passed locally.