feat: add pydantic config models for YAML parsing

- TargetType enum with IMAGE/TEXT variants
- Defaults, TargetConfig, ProjectConfig pydantic models
- infer_target_type() for extension-based type dispatch
- resolve_model() for default/override model resolution
- load_config() using yaml.safe_load + model_validate

bulkgen/config.py

@@ -0,0 +1,82 @@
+"""Pydantic models for bulkgen YAML configuration."""
+
+from __future__ import annotations
+
+import enum
+from pathlib import Path
+from typing import Self
+
+import yaml
+from pydantic import BaseModel, model_validator
+
+
+class TargetType(enum.Enum):
+    """The kind of artifact a target produces."""
+
+    IMAGE = "image"
+    TEXT = "text"
+
+
+IMAGE_EXTENSIONS: frozenset[str] = frozenset({".png", ".jpg", ".jpeg", ".webp"})
+TEXT_EXTENSIONS: frozenset[str] = frozenset({".md", ".txt"})
+
+
+class Defaults(BaseModel):
+    """Default model names, applied when a target does not specify its own."""
+
+    text_model: str = "mistral-large-latest"
+    image_model: str = "flux-pro"
+
+
+class TargetConfig(BaseModel):
+    """Configuration for a single build target."""
+
+    prompt: str
+    model: str | None = None
+    inputs: list[str] = []
+    reference_image: str | None = None
+    control_images: list[str] = []
+    width: int | None = None
+    height: int | None = None
+
+
+class ProjectConfig(BaseModel):
+    """Top-level configuration parsed from ``<name>.bulkgen.yaml``."""
+
+    defaults: Defaults = Defaults()
+    targets: dict[str, TargetConfig]
+
+    @model_validator(mode="after")
+    def _validate_non_empty_targets(self) -> Self:
+        if not self.targets:
+            msg = "At least one target must be defined"
+            raise ValueError(msg)
+        return self
+
+
+def infer_target_type(target_name: str) -> TargetType:
+    """Infer whether a target produces an image or text from its file extension."""
+    suffix = Path(target_name).suffix.lower()
+    if suffix in IMAGE_EXTENSIONS:
+        return TargetType.IMAGE
+    if suffix in TEXT_EXTENSIONS:
+        return TargetType.TEXT
+    msg = f"Cannot infer target type for '{target_name}': unsupported extension '{suffix}'"
+    raise ValueError(msg)
+
+
+def resolve_model(target_name: str, target: TargetConfig, defaults: Defaults) -> str:
+    """Return the effective model for a target (explicit or default by type)."""
+    if target.model is not None:
+        return target.model
+    target_type = infer_target_type(target_name)
+    if target_type is TargetType.IMAGE:
+        return defaults.image_model
+    return defaults.text_model
+
+
+def load_config(config_path: Path) -> ProjectConfig:
+    """Load and validate a ``.bulkgen.yaml`` file."""
+    with config_path.open() as f:
+        raw = yaml.safe_load(f)
+    return ProjectConfig.model_validate(raw)