Models can be deleted directly... but they shouldn't be

Nobody has claimed this yet.

Not sure why I never noticed this before, but we have this method:

https://github.com/luckyframework/avram/blob/80311556f0cc2b1186c88c97a74a6494ad87f48e/src/avram/primary_key_methods.cr#L75-L77

This means you can do this

```
user = UserQuery.find(1)
user.delete
SaveUser.update!(user)
```

The issue here is that this sort of breaks the repository pattern, and also circumvents the whole point of Delete operations. You can't run any other SQL directly against a model, so deleting a model directly should also be restricted.

I didn't look too far in to it, but it's possible this existed before we added DeleteOperations... In any case, we should deprecate the direct use of calling `model.delete`, then later remove it so you have to use a DeleteOperation instead (or bulk query).

Start with src/avram/primary_key_methods.cr at lines 75-77 and trace how model.delete relates to DeleteOperations and bulk queries. Determine the deprecation and eventual removal scope, then verify that deletion is restricted to the intended operation APIs without breaking existing behavior.

Written by the indexing model from the issue text.