CLI 0.19.0 — Switch between dev and stable release channels

mf update can change the installed binary’s release channel and remember the choice for future checks and updates.

Highlights

• Explicit channels. Use mf update --channel dev for pre-release builds or --channel stable for production builds.
• Remembered selection. The machine stores its choice in ~/.manyfold/update-channel.json.
• Safe preview. mf update --check reports the selected channel without changing the saved preference.
• Consistent naming. Installer and daemon status use dev and stable; staging remains an accepted legacy alias.