album update -d with --clear-description silently drops the description #41

Symptom

AlbumCommand::Update declares --description/-d and --clear-description with no
conflicts_with, so giving both parses happily and albums::update takes the clearing branch:

description: if clear_description { Some(None) } else { description.map(|d| Some(d.to_string())) },

The description the person asked for is discarded without a word, exit 0. This is #32's
contradiction in another command — edit has the guard (#[arg(long, short = 'd', conflicts_with = "clear_description")] on EditArgs::description) and album update does not.

Reproduction

imogen album update Holiday -d "new text" --clear-description

Accepted; the album ends with no description.

Expected

The pair refuses to parse, as it does on edit: conflicts_with = "clear_description" on
AlbumCommand::Update's description, with a case in cli.rs's parse tests asserting both the
refusal and that each flag alone still parses.

Affected repos

Only imogen-cli. Nothing on the wire changes.

Context

Named in #38's Context section and confirmed while fixing it; left out of that PR because it is a
different defect from the album-reference lookup.