Use `strings.Builder` to return CLI command output instead of printing

[digitalghost-dev]

Refactoring CLI command functions to return strings using strings.Builder() rather than printing directly with fmt.Println().

Benefits

• Separation of concerns: Command logic is now decoupled from output/display logic.
• Testability: Returned strings can be easily tested without capturing stdout.
• Reusability: Output can be logged, stored, or processed further.
• Scalability: Supports flexible output handling in future (e.g., piping, logging, GUI).