fix(config): reject non-yaml save paths

Co-Authored-By: Virgil <virgil@lethean.io>

config.go

@@ -253,6 +253,13 @@ func Load(m coreio.Medium, path string) (map[string]any, error) {
 // Save writes configuration data to a YAML file at the given path.
 // It ensures the parent directory exists before writing.
 func Save(m coreio.Medium, path string, data map[string]any) error {
+	switch ext := strings.ToLower(filepath.Ext(path)); ext {
+	case "", ".yaml", ".yml":
+		// These paths are safe to treat as YAML destinations.
+	default:
+		return coreerr.E("config.Save", "unsupported config file type: "+path, nil)
+	}
+
 	out, err := yaml.Marshal(data)
 	if err != nil {
 		return coreerr.E("config.Save", "failed to marshal config", err)

config_test.go

@@ -312,6 +312,28 @@ func TestSave_Good(t *testing.T) {
 	assert.Contains(t, content, "key: value")
 }
 
+func TestSave_UnsupportedPath_Bad(t *testing.T) {
+	m := coreio.NewMockMedium()
+
+	err := Save(m, "/tmp/test/config.json", map[string]any{"key": "value"})
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "unsupported config file type")
+}
+
+func TestConfig_Commit_UnsupportedPath_Bad(t *testing.T) {
+	m := coreio.NewMockMedium()
+
+	cfg, err := New(WithMedium(m), WithPath("/tmp/test/config.json"))
+	assert.NoError(t, err)
+
+	err = cfg.Set("key", "value")
+	assert.NoError(t, err)
+
+	err = cfg.Commit()
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "unsupported config file type")
+}
+
 func TestConfig_LoadFile_Env(t *testing.T) {
 	m := coreio.NewMockMedium()
 	m.Files["/.env"] = "FOO=bar\nBAZ=qux"