Replaces the single monolithic installer.nix (preserved as
installer_old.nix for reference) with a shared common.nix carrying the
install-script/user/SSH baseline, consumed by per-platform targets
(installer.nix for netboot/ISO, new proxmox-lxc.nix for the Proxmox
LXC-based flow). flake.nix drops the nixos-generators input in favor
of building images directly from each nixosConfiguration's
system.build.isoImage/tarball.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
