Add assets command group for result manifests

[thebiglabasky]

Summary

• Add a new checkly assets command group for check-result and test-session result asset manifests
• Support listing and downloading assets with shared source validation and unified REST models
• Improve terminal UX with source recap, adaptive tables, and an optional tree view for nested asset paths

Demo

https://www.loom.com/share/b63dd3dcb8e446bc96e1637117d6037a

Testing

• Added unit coverage for REST endpoint selection, list rendering, asset selection, overwrite behavior, and download summaries
• Updated help coverage for the new top-level topic