Improve hints around `flag create`

Testers report confusion upon seeing this message:

```
matvei@macstream ~ % flagsmith flag list
No flags.
matvei@macstream ~ % flagsmith flag create
Error: flags exist per environment, so there is nothing to create
To create the feature itself, run `flagsmith feature create `.
```

It seems we can do a better job directing users to `feature create`, e.g.:

```
matvei@macstream ~ % flagsmith flag create
Error: flags can only be changed, not created.
To create a feature, run `flagsmith feature create `.
To change a flag, run `flagsmith flag update `.
```

Locate the implementation of the `flag create` command and the code that produces its current error message. Update the guidance to explain that flags are changed rather than created, point users to `feature create` and `flag update`, and verify the resulting command output.

索引モデルが issue の本文から書いたものです。