chore: refactor flag_loader

[minhd-vu]

Description

• Remove flag_loader package to the root directory
• Combine some logic to improve flag parsing

Jira / Linear Tickets

• DVT-000

Testing

• Test A
• Test B

[minhd-vu]

Cobra Best Practices Research

I've done some research on Cobra best practices to improve our CLI development. Here are the key findings:

Flag Sharing Patterns

• Struct with AddFlags() method is the most common pattern in popular projects (kubectl, Docker CLI, Helm)
• Allows sharing flags horizontally across unrelated commands
• Better than PersistentFlags for non-parent-child relationships

Viper + Cobra Integration

• Configuration precedence: Flags > Environment Variables > Config File > Defaults
• Critical timing: Bind flags in PersistentPreRunE (after parsing, before execution), NOT in init()
• Always read from Viper after binding, never from flag variables
• Use mapstructure tags to map dash-containing flag names (e.g., --write-to-tx) to Go struct fields (e.g., WriteToTx)

Key Pattern for Viper

PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
    return viper.BindPFlags(cmd.Flags())  // Magic happens here
}

Resources

• Real-world examples from kubernetes/kubectl, docker/cli, helm/helm
• Flag sharing: Struct + AddFlags() method pattern is industry standard
• Viper unmarshalling: Use mapstructure tags for any flag name → struct field mapping

[jhkimqd]

+1 lgtm

[jhkimqd]

Let's merge #822 into minhd-vu/flag-loader first then we could merge this into main - wdyt?