BaseModel class abstract error

Nobody has claimed this yet.

If you create your own BaseModel, but forget to make it `abstract`, you can end up with an error like this:

```crystal
class MyBaseModel < Avram::Model
macro default_columns
primary_key id : Int16
timestamps
end
end
```

```
Error in lib/avram/src/avram/model.cr:16: abstract `def Avram::Model#id()` must be implemented by Primetime::PrimetimeBaseModel

abstract def id
^~
```

The error is correct, but it doesn't make sense on why you're getting it since `primary_key id` does define a `id` method. To fix this, the `MyBaseModel` just needs to be `abstract`.

```crystal
abstract class MyBaseModel < Avram::Model
```

I'm not sure how we can fix this, but a better error message would be nice here.

Start by reproducing the MyBaseModel example and inspect lib/avram/src/avram/model.cr around the abstract id declaration. Trace why primary_key id is not reflected in the compiler diagnostic when the model is not abstract. Done means the error clearly explains the required abstract declaration or otherwise gives actionable guidance.

Written by the indexing model from the issue text.