Keep the clone upgrade button visible when the engine reports it unavailable

Related to #755.

The clone page hid the "Upgrade clone" button whenever /status reported cloneUpgrade.available: false. That is the state of every instance without provision.pgUpgradeImage set, so a stock 4.2 install looked like it did not ship the feature.

Changes:

• MainStore.isUpgradeSupported (ui/packages/shared/pages/Clone/stores/Main.ts) now gates only on the upgradeClone endpoint existing and on the engine reporting cloneUpgrade at all. An engine older than the action and the Platform app still hide the button.
• New upgradeUnavailableReason getter returns the engine's reason when available is false.
• The clone page renders the button disabled with that reason in a Tooltip. A span wraps the button since MUI sets pointer-events: none on a disabled button and the tooltip would never open.
• README describes the disabled state.

UpgradeCloneModal still mounts only when upgradeTargetVersion is set, unchanged.