Cherry-picked the real changes from all current-system-* branches onto main. Skipped empty WIP snapshot commits.
Commits:
- lightbulb changes - home-assistant, avahi module, host tweaks
- move steam into it's own module - steam moved from megatron to shared module
- more clockwork bs - flake.nix, base module, ssh, helium pkg
- nautilus - graphicalSession.nix for callie
Conflicts resolved in megatron/default.nix.