[CLI] stealth mode not compatible with MacOS

When you try to use stealth mode on a Mac, the dependency on `xsel` is not met:

```
$ cht.sh --shell python
DEPENDENCY: please install "xsel" for "copy"
type 'help' for the cht.sh shell help
```

`xsel` is not available on MacOS and even if, it wouldn't do what's needed.
Likely it's `pbcopy/pbpaste` which should be used instead.

Start with the code path exercised by `cht.sh --shell python` and locate where stealth mode checks for `xsel` and performs copy or paste. Compare the existing behavior with macOS `pbcopy` and `pbpaste`; done means stealth mode no longer reports the missing `xsel` dependency on MacOS and copy/paste works there.

Written by the indexing model from the issue text.