Function add 

pub fn add(
    module: &str,
    manifest_path: &Path,
) -> Result<AddOutcome, Vec<DoctorFailure>>

fz add <module>: adds a module to the manifest’s desired composition and nothing else — it never deploys, never provisions and never touches a database. Adding a module the manifest already lists is a no-op that succeeds with changed: false.

Errors

Coded failures when the manifest cannot be read or resolved, the slug is not in the catalog, or the manifest cannot be written back.