feat(config): add reflection-based configFields framework

cmd/config_fields.go

@@ -0,0 +1,55 @@
+package cmd
+
+import "reflect"
+
+// ConfigField represents a single leaf field in the Config struct,
+// mapped to its dotted TOML key path.
+type ConfigField struct {
+	Key       string        // dotted key, e.g. "git.auto_commit"
+	Value     any           // current value
+	Default   any           // value from defaultConfig()
+	IsDefault bool          // Value == Default
+	Field     reflect.Value // settable reflect.Value (from cfg pointer)
+	Kind      reflect.Kind  // field type kind
+}
+
+// configFields walks cfg and defaults in parallel, returning a ConfigField
+// for every leaf field. Keys are built from TOML struct tags.
+func configFields(cfg, defaults *Config) []ConfigField {
+	var fields []ConfigField
+	walk(reflect.ValueOf(cfg).Elem(), reflect.ValueOf(defaults).Elem(), "", &fields)
+	return fields
+}
+
+func walk(cv, dv reflect.Value, prefix string, out *[]ConfigField) {
+	ct := cv.Type()
+	for i := 0; i < ct.NumField(); i++ {
+		sf := ct.Field(i)
+		tag := sf.Tag.Get("toml")
+		if tag == "" || tag == "-" {
+			continue
+		}
+
+		key := tag
+		if prefix != "" {
+			key = prefix + "." + tag
+		}
+
+		cfv := cv.Field(i)
+		dfv := dv.Field(i)
+
+		if sf.Type.Kind() == reflect.Struct {
+			walk(cfv, dfv, key, out)
+			continue
+		}
+
+		*out = append(*out, ConfigField{
+			Key:       key,
+			Value:     cfv.Interface(),
+			Default:   dfv.Interface(),
+			IsDefault: reflect.DeepEqual(cfv.Interface(), dfv.Interface()),
+			Field:     cfv,
+			Kind:      sf.Type.Kind(),
+		})
+	}
+}