The console log finally showed the real error behind the GRUB-menu hang:
error: prohibited by secure boot policy.
error: bad shim signature.
Failed to boot both default and fallback entries.
Incus defaults VMs to security.secureboot=true. A Debian cloud image
whose shim is signed with a key this host's OVMF does not trust then
fails signature verification, the kernel never loads, and the VM sits
at the GRUB menu forever — which is exactly the 5-min agent timeout on
every box. It worked in runs 11–15 on the old cached image and broke
the moment --purge-storage re-downloaded a build with a different shim.
security.secureboot=false on VM launch (cmd_new, and the drill's legacy
box). Secure Boot inside a throwaway box is not part of its threat
model — the VM boundary is — and off, it boots reliably across image
rebuilds. Container mode has no firmware and is unaffected.
Bare repro that isolated it: 'incus launch images:debian/13/cloud x --vm'
alone reproduced the hang, proving it was never the 0.5.0 code.
912e0621ca