Function cmd_describe 

pub fn cmd_describe(
    registry: &dyn RegistryProvider,
    module_id: &str,
    explicit_format: Option<&str>,
) -> Result<String, DiscoveryError>

Execute the describe subcommand logic.

Returns Ok(String) with the formatted output on success. Returns Err(DiscoveryError) on invalid module ID or module not found.

Exit code mapping for the caller:

• DiscoveryError::InvalidModuleId → exit 2
• DiscoveryError::ModuleNotFound → exit 44