feat: add output package with Formatter interface (#537)

* docs: add RFC-004 for CLI (cobra) and output design
* feat: add output package with Formatter interface and BrowserData.Each
* fix: golangci config array syntax + add output package tests
* refactor: encapsulated Output as Writer, collect-then-write pattern
* refactor: unified row type with reflection-based CSV/JSON output
* fix: ProfileName empty guard, writeFile close error check, sync RFC-004

output/reflect.go

@@ -0,0 +1,89 @@
+package output
+
+import (
+	"fmt"
+	"reflect"
+	"strconv"
+	"strings"
+	"time"
+)
+
+var timeType = reflect.TypeOf(time.Time{})
+
+// structCSVHeader extracts CSV column names from a struct's csv tags.
+func structCSVHeader(v any) []string {
+	t := reflect.TypeOf(v)
+	headers := make([]string, 0, t.NumField())
+	for i := 0; i < t.NumField(); i++ {
+		name := tagName(t.Field(i), "csv")
+		if name == "" {
+			continue
+		}
+		headers = append(headers, name)
+	}
+	return headers
+}
+
+// structCSVRow converts a struct's field values to CSV string values,
+// including only fields that have a csv tag.
+func structCSVRow(v any) []string {
+	val := reflect.ValueOf(v)
+	t := val.Type()
+	row := make([]string, 0, t.NumField())
+	for i := 0; i < t.NumField(); i++ {
+		if tagName(t.Field(i), "csv") == "" {
+			continue
+		}
+		row = append(row, fieldToString(val.Field(i)))
+	}
+	return row
+}
+
+// tagName extracts the tag value for the given key from a struct field.
+// Uses Lookup (not Get) to distinguish "no tag" from "empty tag".
+// Returns "" if the tag is absent, empty, or "-".
+func tagName(f reflect.StructField, key string) string {
+	tag, ok := f.Tag.Lookup(key)
+	if !ok || tag == "-" {
+		return ""
+	}
+	if idx := strings.IndexByte(tag, ','); idx != -1 {
+		tag = tag[:idx]
+	}
+	if tag == "" {
+		return ""
+	}
+	return tag
+}
+
+func fieldToString(v reflect.Value) string {
+	// Check time.Time before kind switch since it's a struct.
+	if v.Type() == timeType {
+		t, _ := v.Interface().(time.Time)
+		return formatTime(t)
+	}
+	switch v.Kind() {
+	case reflect.String:
+		return v.String()
+	case reflect.Bool:
+		return formatBool(v.Bool())
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		return strconv.FormatInt(v.Int(), 10)
+	default:
+		return fmt.Sprintf("%v", v.Interface())
+	}
+}
+
+func formatTime(t time.Time) string {
+	if t.IsZero() {
+		return ""
+	}
+	return t.Format(time.RFC3339)
+}
+
+func formatBool(b bool) string {
+	if b {
+		return "true"
+	}
+	return "false"
+}