scripting: add an install command for macOS guests

[meisenbarth]

Follow-up to #6092 and #7862.

What is blocked

VMDisplayAppleWindowController.windowDidLoad shows a confirmation sheet before calling requestInstallVM, so a macOS install cannot be driven from a script without someone clicking OK.

This does not remove the sheet

That is deliberate. It guards a genuinely destructive action — it erases the primary drive — and a GUI user who opens a VM that happens to carry an IPSW should keep being asked.

It is also structurally unavailable to a script: showConfirmAlert calls beginSheetModal(for: window!), so it needs a window. The scripted start verb calls data.run(...), which loads the window controller — that is what raises the sheet in the first place.

So this adds a separate verb that goes straight to installVM(with:) and never opens a window. The confirmation moves rather than disappearing: asking for install is the confirmation, which is why the sdef entry says DESTRUCTIVE in as many words. This seemed to match the guidance in CONTRIBUTING.md about putting things most users do not need into the scripting interface rather than the UI.

tell application "UTM"
  install virtual machine "my-macos-vm"
end tell

It also clears the IPSW on success

As the GUI does in didCompleteInstallation. That clearing lives in the window controller's delegate callback, and a scripted install has no window — so without it the IPSW outlives the install, and the next start that does open a display window asks "Would you like to install macOS?" over a guest that is already installed. Measured before the fix: the registry still held the IPSW after a completed scripted install.

Two guards

installVM returns silently when the VM is not stopped, so calling it unguarded would report success having done nothing. Both cases were exercised:

call	result
install a Linux VM	"This virtual machine has no IPSW recovery image configured, so there is nothing to install from."
install a running VM	"The virtual machine must be stopped before this operation can be performed."

Testing

Built against main from a clean baseline, Xcode 26.6, macOS arm64. This branch applies and builds independently of #7862.

check	result
baseline / this branch alone	clean, 0 errors
install resolves as a verb	yes — decompiles as install virtual machine
no confirmation sheet at any point	yes
install runs to completion	yes — 21 GB written, framework returned success
IPSW cleared afterwards	yes — the registry entry no longer carries it
subsequent start	opens a display window, no reinstall prompt, guest boots to Setup Assistant
errors reach the caller	yes

Install timeline, macOS 26.6.2 (25G83):

00:00  started, no sheet
00:51  2.4 GB
02:31  19 GB
03:21  21 GB — install returned OK

No generated-code changes are needed for this one.