gplay metadata sync (listings + images)

Manage the store front: per-locale listing text and images, kept as an on-disk tree and reconciled with Play. Shared conventions (auth, --package, output, exit codes) are in gplay-cli-usage.

The sync model

gplay treats a local ./metadata tree (<locale>/<field>.txt, plus image files) as the desired state and reconciles it with Play (ADR-0011). The loop:

pull  →  edit on disk  →  validate (offline)  →  apply --dry-run  →  apply --confirm