Add NixOS configuration for ringtail workstation (#207)

## Summary
- NixOS flake for ringtail (gaming/compute workstation, RTX 4080) in `nixos/ringtail/`
- Declarative disk partitioning via disko (GPT, 512M EFI + ext4 root on NVMe)
- NVIDIA proprietary drivers, sway/Wayland desktop, greetd, PipeWire, Steam
- Tailscale integration for tailnet connectivity
- Ansible playbook + `mise run provision-ringtail` for ongoing management
- Pulumi auth key (`tag:homelab`, `tag:blumeops`) for tailnet bootstrap

## Deployment Order
1. **Merge PR**
2. `pulumi up` in tailscale stack → creates auth key
3. Retrieve auth key: `pulumi stack output ringtail_authkey --show-secrets`
4. On ringtail NixOS installer:
   - `nix run github:nix-community/disko -- --mode disko /tmp/disk-config.nix` (or from cloned repo)
   - `nixos-install --flake github:eblume/blumeops?dir=nixos/ringtail#ringtail`
5. Reboot, `tailscale up --auth-key=<key>`
6. Verify: `tailscale status`, SSH from gilbert

## Test plan
- [ ] Review NixOS configuration for completeness
- [ ] Verify disko partition layout matches ringtail hardware
- [ ] Run `pulumi preview` for tailscale stack
- [ ] Install NixOS on ringtail
- [ ] Confirm tailscale connectivity
- [ ] Confirm sway desktop works
- [ ] Test `mise run provision-ringtail` for ongoing management

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/207

nixos/ringtail/flake.nix

@@ -0,0 +1,23 @@
+{
+  description = "NixOS configuration for ringtail (gaming/compute workstation)";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+    disko = {
+      url = "github:nix-community/disko";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { nixpkgs, disko, ... }: {
+    nixosConfigurations.ringtail = nixpkgs.lib.nixosSystem {
+      system = "x86_64-linux";
+      modules = [
+        disko.nixosModules.disko
+        ./disk-config.nix
+        ./hardware-configuration.nix
+        ./configuration.nix
+      ];
+    };
+  };
+}