Function cmd_list 

pub fn cmd_list(
    registry: &dyn RegistryProvider,
    tags: &[&str],
    explicit_format: Option<&str>,
) -> Result<String, DiscoveryError>

Execute the list subcommand logic.

Returns Ok(String) with the formatted output on success. Returns Err(DiscoveryError) on invalid tag format.

Exit code mapping for the caller: DiscoveryError::InvalidTag → exit 2.