Spot Shell (v0.20.0)

Propagate spotsh tags to AMIs and infer launch user

Copy the `spotsh.user` and `spotsh.os` tags from the source instance
onto newly-created AMIs, and update launch handling to treat `--user` as
explicit input while best-effort deriving it from AMI tags when
launching by AMI. Preserve the existing validation failure when no user
can be determined, and ensure image creation uses the selected instance
ID when one is chosen interactively.

Co-authored-by: Octium Agent <agent@octium.dev>