Complete inbox CLI implementation

internal/cli/inbox/body.go

@@ -0,0 +1,21 @@
+package inbox
+
+import (
+	"fmt"
+	"os"
+)
+
+func resolveBodyValue(body, bodyFile string) (string, error) {
+	if body != "" && bodyFile != "" {
+		return "", fmt.Errorf("body and body-file are mutually exclusive")
+	}
+	if bodyFile == "" {
+		return body, nil
+	}
+
+	content, err := os.ReadFile(bodyFile)
+	if err != nil {
+		return "", fmt.Errorf("read body file %q: %w", bodyFile, err)
+	}
+	return string(content), nil
+}