Running niri and umbriel side by side in a NixOS flake: only one may own tty1 autostart

When adding a second Wayland compositor module (e.g. restoring a niri-flake module next to umbriel), each home module tends to ship its own programs.fish.loginShellInit block that does exec <session> on /dev/tty1. Both strings concatenate, so the first exec wins and the order is not obvious. Keep the autostart only in the default compositor and delete it from the other. Also set the greeter default session (noctalia-greeter: [session] default = "Niri") and the vmVariant greetd initial_session command. Restoring a removed flake input with nix flake lock re-locked niri-flake at the same revs the old lock had, because nested inputs resolved to the same commits.