Add GitHub Actions smoke test workflow

[keepithandy]

Goal

Add a tiny CI workflow so Depth Engine automatically runs its smoke tests on pushes and pull requests.

Current behavior

The repo has smoke scripts and a manual test checklist, but there is no automated GitHub Actions workflow yet.

Proposed work

• Add a minimal workflow under .github/workflows/.
• Run node smoke_depth_engine_core.mjs.
• Run node smoke_rat_cellar_content.mjs.
• Keep the workflow simple and dependency-free.
• Do not add a package manager or build system.

Acceptance checks

• Pushes to main run the smoke workflow.
• Pull requests run the smoke workflow.
• Both existing smoke scripts pass in CI.
• The project still opens directly from index.html without install steps.

Guardrails

This is automation polish only. Do not introduce npm, bundling, frameworks, or a build step in this issue.