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

ansible/inventory/hosts.yml

@@ -5,6 +5,9 @@ all:
       hosts:
         indri:
           ansible_host: indri
+        ringtail:
+          ansible_host: ringtail
+          ansible_user: eblume
     workstations:
       hosts:
         gilbert: