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/validators.py

@@ -0,0 +1,159 @@
+"""Validation functions for host configuration."""
+
+import re
+from pathlib import Path
+from typing import Optional
+
+
+def validate_hostname_format(hostname: str) -> None:
+    """
+    Validate hostname format according to RFC 1123.
+
+    Args:
+        hostname: Hostname to validate
+
+    Raises:
+        ValueError: If hostname format is invalid
+    """
+    # RFC 1123: lowercase, alphanumeric, hyphens, max 63 chars
+    pattern = r"^[a-z0-9]([a-z0-9-]{0,61}[a-z0-9])?$"
+
+    if not re.match(pattern, hostname):
+        raise ValueError(
+            f"Invalid hostname '{hostname}'. "
+            "Must be lowercase alphanumeric with hyphens, "
+            "start and end with alphanumeric, max 63 characters."
+        )
+
+
+def validate_hostname_unique(hostname: str, repo_root: Path) -> None:
+    """
+    Validate that hostname is unique in the repository.
+
+    Args:
+        hostname: Hostname to check
+        repo_root: Path to repository root
+
+    Raises:
+        ValueError: If hostname already exists
+    """
+    # Check if host directory exists
+    host_dir = repo_root / "hosts" / hostname
+    if host_dir.exists():
+        raise ValueError(f"Host directory already exists: {host_dir}")
+
+    # Check if hostname exists in flake.nix
+    flake_path = repo_root / "flake.nix"
+    if flake_path.exists():
+        flake_content = flake_path.read_text()
+        # Look for pattern like "      hostname = "
+        hostname_pattern = rf'^\s+{re.escape(hostname)}\s*='
+        if re.search(hostname_pattern, flake_content, re.MULTILINE):
+            raise ValueError(f"Hostname '{hostname}' already exists in flake.nix")
+
+
+def validate_ip_format(ip: str) -> None:
+    """
+    Validate IP address format with CIDR notation.
+
+    Args:
+        ip: IP address with CIDR (e.g., "10.69.13.50/24")
+
+    Raises:
+        ValueError: If IP format is invalid
+    """
+    if not ip:
+        return
+
+    # Check CIDR notation
+    if "/" not in ip:
+        raise ValueError(f"IP address must include CIDR notation (e.g., {ip}/24)")
+
+    ip_part, cidr_part = ip.rsplit("/", 1)
+
+    # Validate CIDR is /24
+    if cidr_part != "24":
+        raise ValueError(f"CIDR notation must be /24, got /{cidr_part}")
+
+    # Validate IP format
+    octets = ip_part.split(".")
+    if len(octets) != 4:
+        raise ValueError(f"Invalid IP address format: {ip_part}")
+
+    try:
+        octet_values = [int(octet) for octet in octets]
+    except ValueError:
+        raise ValueError(f"Invalid IP address format: {ip_part}")
+
+    # Check each octet is 0-255
+    for i, value in enumerate(octet_values):
+        if not 0 <= value <= 255:
+            raise ValueError(f"Invalid octet value {value} in IP address")
+
+    # Check last octet is 1-254
+    if not 1 <= octet_values[3] <= 254:
+        raise ValueError(
+            f"Last octet must be 1-254, got {octet_values[3]}"
+        )
+
+
+def validate_ip_subnet(ip: str) -> None:
+    """
+    Validate that IP address is in the correct subnet (10.69.13.0/24).
+
+    Args:
+        ip: IP address with CIDR (e.g., "10.69.13.50/24")
+
+    Raises:
+        ValueError: If IP is not in correct subnet
+    """
+    if not ip:
+        return
+
+    validate_ip_format(ip)
+
+    ip_part = ip.split("/")[0]
+    octets = ip_part.split(".")
+
+    # Check subnet is 10.69.13.x
+    if octets[:3] != ["10", "69", "13"]:
+        raise ValueError(
+            f"IP address must be in 10.69.13.0/24 subnet, got {ip_part}"
+        )
+
+
+def validate_ip_unique(ip: Optional[str], repo_root: Path) -> None:
+    """
+    Validate that IP address is not already in use.
+
+    Args:
+        ip: IP address with CIDR to check (None for DHCP)
+        repo_root: Path to repository root
+
+    Raises:
+        ValueError: If IP is already in use
+    """
+    if not ip:
+        return  # DHCP mode, no uniqueness check needed
+
+    # Extract just the IP part without CIDR for searching
+    ip_part = ip.split("/")[0]
+
+    # Check all hosts/*/configuration.nix files
+    hosts_dir = repo_root / "hosts"
+    if hosts_dir.exists():
+        for config_file in hosts_dir.glob("*/configuration.nix"):
+            content = config_file.read_text()
+            if ip_part in content:
+                raise ValueError(
+                    f"IP address {ip_part} already in use in {config_file}"
+                )
+
+    # Check terraform/vms.tf
+    terraform_file = repo_root / "terraform" / "vms.tf"
+    if terraform_file.exists():
+        content = terraform_file.read_text()
+        if ip_part in content:
+            raise ValueError(
+                f"IP address {ip_part} already in use in {terraform_file}"
+            )