Function cmd_completion 

pub fn cmd_completion(
    shell: Shell,
    prog_name: &str,
    cmd: &mut Command,
) -> String

Handler: generate a shell completion script and return it as a String.

For bash, zsh, and fish this produces grouped completion scripts that support _APCORE_GRP-based group filtering (matching the Python reference implementation). Other shells fall back to clap_complete.

shell — the target shell (parsed from clap argument) prog_name — the program name to embed in the script cmd — mutable reference to the root Command (required by clap_complete)