Derive clone upgrade target from the configured image

Scope

Track the API, engine, CLI, and UI changes that derive the clone-upgrade target from the configured upgrade image, validate image-major consistency, advertise availability through instance status, and remove the redundant free-text target-version input.

Parent feature: #755

Implementation MR: !1188 (merged)

Done when

• The server resolves and caches the configured upgrade target safely.
• Explicit image overrides are validated against the target major.
• API/CLI/UI consume the server-advertised plan.
• Tests cover image probing, reload behavior, validation, and clients.
• Independent review and required CI pass.