BaseProvider missing model attribute declaration

[baranylcn]

All provider classes (OpenAIProvider, AnthropicProvider, etc.) store the model name as self.model, but the abstract base class never declares this attribute. Type checkers like mypy can't see it.

Current behavior

# base.py
class BaseProvider(ABC):
    @abstractmethod
    async def run(self, prompt: str, ...) -> ModelResult: ...
    # no `model` attribute

# every provider does this, but base doesn't know about it
class OpenAIProvider(BaseProvider):
    def __init__(self, model: str) -> None:
        self.model = model  # type checker can't verify this

Expected behavior

class BaseProvider(ABC):
    model: str  # declared here

    @abstractmethod
    async def run(self, prompt: str, ...) -> ModelResult: ...

Files to change

skate/providers/base.py - add model: str class-level annotation

That's it. One line.

How to verify

pip install -e ".[dev]"
mypy skate/providers/base.py
pytest tests/

Tests should still pass, mypy should be happy.