pipeline: add testing improvements for branch-based workflows

Implement dual improvements to enable efficient testing of pipeline changes
without polluting master branch:

1. Add --force flag to create-host script
   - Skip hostname/IP uniqueness validation
   - Overwrite existing host configurations
   - Update entries in flake.nix and terraform/vms.tf (no duplicates)
   - Useful for iterating on configurations during testing

2. Add branch support to bootstrap mechanism
   - Bootstrap service reads NIXOS_FLAKE_BRANCH environment variable
   - Defaults to master if not set
   - Uses branch in git URL via ?ref= parameter
   - Service loads environment from /etc/environment

3. Add cloud-init disk support for branch configuration
   - VMs can specify flake_branch field in terraform/vms.tf
   - Automatically generates cloud-init snippet setting NIXOS_FLAKE_BRANCH
   - Uploads snippet to Proxmox via SSH
   - Production VMs omit flake_branch and use master

4. Update documentation
   - Document --force flag usage in create-host README
   - Add branch testing examples in terraform README
   - Update TODO.md with testing workflow
   - Add .generated/ to gitignore

Testing workflow: Create feature branch, set flake_branch in VM definition,
deploy with terraform, iterate with --force flag, clean up before merging.

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

scripts/create-host/create_host.py

@@ -45,6 +45,7 @@ def main(
     memory: int = typer.Option(2048, "--memory", help="Memory in MB"),
     disk: str = typer.Option("20G", "--disk", help="Disk size (e.g., 20G, 50G, 100G)"),
     dry_run: bool = typer.Option(False, "--dry-run", help="Preview changes without creating files"),
+    force: bool = typer.Option(False, "--force", help="Overwrite existing host configuration"),
 ) -> None:
     """
     Create a new NixOS host configuration.
@@ -75,11 +76,20 @@ def main(
 
         config.validate()
         validate_hostname_format(hostname)
-        validate_hostname_unique(hostname, repo_root)
+
+        # Skip uniqueness checks in force mode
+        if not force:
+            validate_hostname_unique(hostname, repo_root)
+            if ip:
+                validate_ip_unique(ip, repo_root)
+        else:
+            # Check if we're actually overwriting something
+            host_dir = repo_root / "hosts" / hostname
+            if host_dir.exists():
+                console.print(f"[yellow]⚠[/yellow]  Updating existing host configuration for {hostname}")
 
         if ip:
             validate_ip_subnet(ip)
-            validate_ip_unique(ip, repo_root)
 
         console.print("[green]✓[/green] All validations passed\n")
 
@@ -96,13 +106,14 @@ def main(
         console.print("\n[bold blue]Generating host configuration...[/bold blue]")
 
         generate_host_files(config, repo_root)
-        console.print(f"[green]✓[/green] Created hosts/{hostname}/default.nix")
-        console.print(f"[green]✓[/green] Created hosts/{hostname}/configuration.nix")
+        action = "Updated" if force else "Created"
+        console.print(f"[green]✓[/green] {action} hosts/{hostname}/default.nix")
+        console.print(f"[green]✓[/green] {action} hosts/{hostname}/configuration.nix")
 
-        update_flake_nix(config, repo_root)
+        update_flake_nix(config, repo_root, force=force)
         console.print("[green]✓[/green] Updated flake.nix")
 
-        update_terraform_vms(config, repo_root)
+        update_terraform_vms(config, repo_root, force=force)
         console.print("[green]✓[/green] Updated terraform/vms.tf")
 
         # Success message