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/row.go

@@ -0,0 +1,47 @@
+package output
+
+import (
+	"encoding/json"
+	"reflect"
+)
+
+// row wraps any entry with browser/profile context for output.
+type row struct {
+	Browser string
+	Profile string
+	entry   any
+}
+
+func (r row) csvHeader() []string {
+	return append([]string{"browser", "profile"}, structCSVHeader(r.entry)...)
+}
+
+func (r row) csvRow() []string {
+	return append([]string{r.Browser, r.Profile}, structCSVRow(r.entry)...)
+}
+
+// MarshalJSON produces flat JSON with browser/profile followed by the entry's fields.
+// Uses reflect.StructOf to dynamically build a struct that json.Marshal handles natively,
+// avoiding manual JSON string concatenation.
+func (r row) MarshalJSON() ([]byte, error) {
+	ev := reflect.ValueOf(r.entry)
+	et := ev.Type()
+
+	fields := make([]reflect.StructField, 0, et.NumField()+2)
+	fields = append(fields,
+		reflect.StructField{Name: "Browser", Type: reflect.TypeOf(""), Tag: `json:"browser"`},
+		reflect.StructField{Name: "Profile", Type: reflect.TypeOf(""), Tag: `json:"profile"`},
+	)
+	for i := 0; i < et.NumField(); i++ {
+		fields = append(fields, et.Field(i))
+	}
+
+	flat := reflect.New(reflect.StructOf(fields)).Elem()
+	flat.Field(0).SetString(r.Browser)
+	flat.Field(1).SetString(r.Profile)
+	for i := 0; i < et.NumField(); i++ {
+		flat.Field(i + 2).Set(ev.Field(i))
+	}
+
+	return json.Marshal(flat.Interface())
+}