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/formatter.go

@@ -0,0 +1,25 @@
+package output
+
+import (
+	"fmt"
+	"io"
+)
+
+// formatter serializes rows to a writer. Unexported — only used by Writer.
+type formatter interface {
+	format(w io.Writer, rows []row) error
+	ext() string
+}
+
+func newFormatter(name string) (formatter, error) {
+	switch name {
+	case "csv":
+		return &csvFormatter{}, nil
+	case "json":
+		return &jsonFormatter{}, nil
+	case "cookie-editor":
+		return &cookieEditorFormatter{}, nil
+	default:
+		return nil, fmt.Errorf("unsupported format: %s", name)
+	}
+}