fix(cli): reference a published sandbox image

[cwing-nvidia]

Summary

Replace stale references to the unpublished python community sandbox with the published base image so users can follow the CLI and SDK examples successfully.

Related Issue

Closes #2888

Changes

• Updated the openshell sandbox help example to use --from base
• Updated the Rust SDK image-reference documentation to show the published base image

Testing

• git diff --check passes
• mise run pre-commit (not run: mise is unavailable in this environment)
• Unit tests added/updated (not applicable: string and documentation examples only)
• E2E tests added/updated (not applicable)

Checklist

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

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