* Enable incus on pureintent, rename lxd.nix, drop unused flake-parts
- Add incus module import to pureintent and bind the UI to its
Tailscale IP (no firewall change needed since tailscale0 is trusted).
- Rename modules/nixos/linux/lxd.nix -> incus.nix since the module
configures virtualisation.incus, and enable the bundled web UI.
- Drop the unused modules/flake-parts/incus-image helper; the
`images:nixos/*` community images cover container/VM launches.
* Move incus module into a directory with a README
The troubleshooting notes used to live as comments in the module; they
belong in docs alongside a quick-start on launching containers/VMs and
configuring the UI listener.
* Expand incus README with VM gotchas
Document the sharp edges hit while bringing up a NixOS VM for the first
time: secureboot, memory/cpu/disk limits (with the error signatures
that point at each), configuring the guest (flakes, firewall), and a
three-step guide to exposing a service from inside.