enh: Propagate SaveMetadata errors during compose updates

[nfebe]

Context

From PR #109 review.

When UpdateComposeFile syncs metadata to service.yml, the return value of SaveMetadata is silently ignored. If saving fails, the compose file is updated but service.yml is stale — an inconsistent state.

Location

internal/docker/discovery.go — UpdateComposeFile(), around the d.SaveMetadata(name, existing) call.

Expected

Propagate the error:

if err := d.SaveMetadata(name, existing); err != nil {
    return fmt.Errorf("failed to sync service metadata: %w", err)
}