Point image release assets at rootcell and drop rootcell-images references

[jimpudar]

Summary

• Switch the default image manifest URL to rootcell-ai/rootcell and update tests/docs accordingly.
• Remove remaining migration language that referenced rootcell-ai/rootcell-images.
• Update the image release workflow so normal releases are marked latest, keeping the default manifest URL current.

Testing

• bun test
• bun run typecheck
• bun run lint
• git diff --check

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