Function create_sub_module 

pub fn create_sub_module(
    ito_path: &Path,
    name: &str,
    parent_module: &str,
    description: Option<&str>,
) -> Result<CreateSubModuleResult, CreateError>

Create a new sub-module directory under an existing parent module.

Allocates the next available sub-module number, creates the sub/SS_name/ directory, and writes a module.md with the given title and optional description.

Returns CreateError::ModuleNotFound when the parent module does not exist, and CreateError::DuplicateSubModuleName when a sub-module with the same name already exists under that parent.