Function add_command 

pub fn add_command(
    path: &Path,
    name: &str,
    description: &str,
) -> Result<AddCommandOutcome, String>

Append a top-level command entry under commands: in the fdl.yml at path if the entry isn’t already declared.

description is written as a description: subfield. Pass an empty string to omit it (the entry then has nothing under it, falling back to the convention-default path: ./<name>/).

Behaviour:

• commands: table present, name absent → append the entry at end of the commands block, AddCommandOutcome::Added.
• commands: present and name already declared → file untouched, AddCommandOutcome::AlreadyPresent.
• commands: absent → append \ncommands:\n name:\n description: ...\n at end of file, AddCommandOutcome::Added.