Steam Cloud is now set up for the full game, and is linked to the same save path as the demo.
This should seamlessly carry over people's saves from the demo, however - if you have local data for the full game that would be overwritten by a previous demo, you should probably make a copy of your save directory before playing, to restore your files after Steam does the first cloud sync.
You can also choose to use local data when Steam prompts you, if you just want to keep what you have. Here are the save directory paths:
Windows userdata path: Users\\<username>\\AppData\\Roaming\\godot\\app_userdata\\Atomcraft
Linux Path on Proton for Atomcraft (full game): ~/.local/share/Steam/steamapps/compatdata/2803490/pfx/drive_c/users/steamuser/AppData/Roaming/godot/app_userdata/Atomcraft/
Linux Path on Proton for Atomcraft (demo): ~/.local/share/Steam/steamapps/compatdata/3649520/pfx/drive_c/users/steamuser/AppData/Roaming/godot/app_userdata/Atomcraft/
Linux Path on Proton for Atomcraft (beta): ~/.local/share/Steam/steamapps/compatdata/3982270/pfx/drive_c/users/steamuser/AppData/Roaming/godot/app_userdata/Atomcraft/
NOTE: if it overwrote the directory, one possible fix that was mentioned in the forum is - if you go to the userdata directory, open Properties on it and go to the Previous Versions tab, you may be able to revert to a more recent version