scripts: add create-host tool for automated host configuration generation

Implements Phase 2 of the automated deployment pipeline.

This commit adds a Python CLI tool that automates the creation of NixOS host
configurations, eliminating manual boilerplate and reducing errors.

Features:
- Python CLI using typer framework with rich terminal UI
- Comprehensive validation (hostname format/uniqueness, IP subnet/uniqueness)
- Jinja2 templates for NixOS configurations
- Automatic updates to flake.nix and terraform/vms.tf
- Support for both static IP and DHCP configurations
- Dry-run mode for safe previews
- Packaged as Nix derivation and added to devShell

Usage:
  create-host --hostname myhost --ip 10.69.13.50/24

The tool generates:
- hosts/<hostname>/default.nix
- hosts/<hostname>/configuration.nix
- Updates flake.nix with new nixosConfigurations entry
- Updates terraform/vms.tf with new VM definition

All generated configurations include full system imports (monitoring, SOPS,
autoupgrade, etc.) and are validated with nix flake check and tofu validate.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

scripts/create-host/templates/default.nix.j2

@@ -0,0 +1,5 @@
+{ ... }: {
+  imports = [
+    ./configuration.nix
+  ];
+}