feat: add abstract Provider base class

- Defines async generate() interface for generation backends
- Takes target config, resolved prompt/model, and project dir

bulkgen/providers/__init__.py

@@ -0,0 +1,31 @@
+"""Provider abstraction for AI generation backends."""
+
+from __future__ import annotations
+
+import abc
+from pathlib import Path
+
+from bulkgen.config import TargetConfig
+
+
+class Provider(abc.ABC):
+    """Abstract base for generation providers."""
+
+    @abc.abstractmethod
+    async def generate(
+        self,
+        target_name: str,
+        target_config: TargetConfig,
+        resolved_prompt: str,
+        resolved_model: str,
+        project_dir: Path,
+    ) -> None:
+        """Generate the target artifact and write it to *project_dir / target_name*.
+
+        Args:
+            target_name: Output filename (relative to project_dir).
+            target_config: The parsed target configuration.
+            resolved_prompt: The fully resolved prompt text.
+            resolved_model: The resolved model name.
+            project_dir: The project working directory.
+        """