Add config loading and dry-run validation

internal/config/load.go

@@ -0,0 +1,29 @@
+package config
+
+import (
+	"fmt"
+	"os"
+
+	"gopkg.in/yaml.v3"
+)
+
+func LoadFile(path string) (Config, error) {
+	file, err := os.Open(path)
+	if err != nil {
+		return Config{}, fmt.Errorf("load config %q: %w", path, err)
+	}
+	defer file.Close()
+
+	var cfg Config
+	decoder := yaml.NewDecoder(file)
+	decoder.KnownFields(true)
+	if err := decoder.Decode(&cfg); err != nil {
+		return Config{}, fmt.Errorf("parse config %q: %w", path, err)
+	}
+
+	ApplyDefaults(&cfg)
+	if err := Validate(cfg); err != nil {
+		return Config{}, fmt.Errorf("validate config %q: %w", path, err)
+	}
+	return cfg, nil
+}