update: switch Terraform to NixOS template workflow

- 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

nixos/template-base/README.md

@@ -0,0 +1,27 @@
+# NixOS Proxmox Template Base
+
+This folder contains a minimal NixOS base config you can copy into a new
+template VM build.
+
+## Files
+
+- `flake.nix`: pins `nixos-24.11` and exposes one host config.
+- `configuration.nix`: base settings for Proxmox guest use.
+
+## Before first apply
+
+1. Replace `REPLACE_WITH_YOUR_SSH_PUBLIC_KEY` in `configuration.nix`.
+2. Add `hardware-configuration.nix` from the VM install:
+   - `nixos-generate-config --root /`
+   - copy `/etc/nixos/hardware-configuration.nix` next to `configuration.nix`
+
+## Build/apply example inside the VM
+
+```bash
+sudo nixos-rebuild switch --flake .#template
+```
+
+## Notes
+
+- This is intentionally minimal and avoids cloud-init assumptions.
+- If you want host-specific settings, create additional modules and import them.