refactor: make MarkTaskDone and similar functions return errors

[LudoLoops]

Issue

Functions like MarkTaskDone, MarkTaskInProgress print errors directly instead of returning them.

func MarkTaskDone(cfg config.Config, taskTitle string) {  // ← Returns void
    // ...
    if foundTask == nil {
        fmt.Printf("Task '%s' not found.\n", taskTitle)  // Prints directly
        return
    }
    // ...
}

Problem

• Caller cannot tell if operation succeeded
• Error messages go to stdout, not stderr
• Caller cannot make decisions based on success/failure
• Inconsistent with Go error handling patterns

Solution

Change signatures to return error:

func MarkTaskDone(cfg config.Config, taskTitle string) error {
    // ...
    if foundTask == nil {
        return fmt.Errorf("task not found: %s", taskTitle)
    }
    // ...
    return nil
}

Update all callers to check errors.

Affected Functions

• MarkTaskDone
• MarkTaskInProgress
• MarkTaskInProgressByTitle

Benefits

• Proper Go error handling
• Caller has control over error messages
• Enables better error handling in CLI

Impact

🟡 Medium - Requires caller updates