forget: add confirmation prompt

It would be good to get a confirmation prompt when using the `forget` command, like so:
```
keep 1 snapshots:
Snapshot ID Time Archives Keep Reason
-----------------------------------------------------------------------------------------------------------
7ec0121fb3327b05cbdcfb0574e3b9f8 2026-07-09 19:02:38 test.tar.zst weekly snapshot
-----------------------------------------------------------------------------------------------------------

remove 2 snapshots:
Snapshot ID Time Archives
-----------------------------------------------------------------------------------------
3e62021b24e190fe5dd4a15e8d6fe965 2026-07-08 12:14:43 test.tar.gz
213837ab57cfa506af59ea70bf525674 2026-07-08 16:03:18 test.tar
-----------------------------------------------------------------------------------------

Press y/n to apply or dismiss these changes:
```

Start at the implementation and entry point for the `forget` command, then trace where the snapshots to keep and remove are displayed. Add an interactive y/n confirmation before applying the changes, with dismissal leaving them unapplied; verify both responses using the command's existing tests or manual CLI runs.

Written by the indexing model from the issue text.