Add a first-boot tailscale enrollment service to the NixOS template and wire terraform-apply to inject TS auth key at runtime from secrets, so keys are not baked into templates or repo files.
- Point clone_template to nixos-template and trim cloud-init to Nix-safe hostname/DNS only
- Remove SSH/Tailscale cloud-init variables and workflow secret dependencies
- Add reusable NixOS template-base config with bootloader, Tailscale, fish, and utility packages
