fix(helm): vendor chart dependencies before release packaging

[TaylorMutch]

Summary

Fix the Helm OCI release packaging path that failed in https://github.com/NVIDIA/OpenShell/actions/runs/26617316250/job/78436974753. The release action now vendors chart dependencies in the staged chart before running helm package, so the PostgreSQL subchart is included without committing generated chart artifacts.

Related Issue

References pipeline failure: https://github.com/NVIDIA/OpenShell/actions/runs/26617316250/job/78436974753

Changes

• Added a Build chart dependencies step to the reusable Helm OCI release action.
• Run helm dependency build against the temporary staged chart before dev, public, and SHA-pinned chart packaging.

Testing

• helm dependency build + helm package against a temporary chart copy
• helm lint against the staged chart
• mise run pre-commit passes
• Unit tests added/updated (not applicable)
• E2E tests added/updated (not applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)