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/csv.go

@@ -0,0 +1,28 @@
+package output
+
+import (
+	"encoding/csv"
+	"io"
+)
+
+type csvFormatter struct{}
+
+func (f *csvFormatter) ext() string { return "csv" }
+
+func (f *csvFormatter) format(w io.Writer, rows []row) error {
+	if len(rows) == 0 {
+		return nil
+	}
+
+	cw := csv.NewWriter(w)
+	if err := cw.Write(rows[0].csvHeader()); err != nil {
+		return err
+	}
+	for _, r := range rows {
+		if err := cw.Write(r.csvRow()); err != nil {
+			return err
+		}
+	}
+	cw.Flush()
+	return cw.Error()
+}