fix: make miro testdir resolution explicit based on config

internal/config/config.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"os"
@@ -13,10 +14,6 @@ type Config struct {
 }
 
 type tomlConfig struct {
-	Miro tomlMiroConfig `toml:"miro"`
-}
-
-type tomlMiroConfig struct {
 	TestDir string `toml:"test_dir"`
 }
 
@@ -24,15 +21,37 @@ type tomlMiroConfig struct {
 func ReadConfig(path string) (Config, error) {
 	var raw tomlConfig
 
-	_, err := toml.DecodeFile(path, &raw)
+	meta, err := toml.DecodeFile(path, &raw)
 	if err != nil {
 		if errors.Is(err, os.ErrNotExist) {
 			return Config{}, err
 		}
 		return Config{}, fmt.Errorf("failed to read %s: %v", path, err)
 	}
+	if !meta.IsDefined("test_dir") {
+		return Config{}, fmt.Errorf("failed to read %s: missing required test_dir", path)
+	}
+	if raw.TestDir == "" {
+		return Config{}, fmt.Errorf("failed to read %s: empty test_dir", path)
+	}
 
 	return Config{
-		TestDir: raw.Miro.TestDir,
+		TestDir: raw.TestDir,
 	}, nil
 }
+
+// WriteConfig writes miro.toml.
+func WriteConfig(path string, cfg Config) error {
+	if cfg.TestDir == "" {
+		return errors.New("empty test_dir")
+	}
+
+	var buf bytes.Buffer
+	if err := toml.NewEncoder(&buf).Encode(tomlConfig{
+		TestDir: cfg.TestDir,
+	}); err != nil {
+		return fmt.Errorf("failed to encode %s: %v", path, err)
+	}
+
+	return os.WriteFile(path, buf.Bytes(), 0o644)
+}