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>

hosts/template2/bootstrap.nix

@@ -24,8 +24,12 @@ let
       echo "Network connectivity confirmed"
       echo "Fetching and building NixOS configuration from flake..."
 
+      # Read git branch from environment, default to master
+      BRANCH="''${NIXOS_FLAKE_BRANCH:-master}"
+      echo "Using git branch: $BRANCH"
+
       # Build and activate the host-specific configuration
-      FLAKE_URL="git+https://git.t-juice.club/torjus/nixos-servers.git#''${HOSTNAME}"
+      FLAKE_URL="git+https://git.t-juice.club/torjus/nixos-servers.git?ref=$BRANCH#''${HOSTNAME}"
 
       if nixos-rebuild boot --flake "$FLAKE_URL"; then
         echo "Successfully built configuration for $HOSTNAME"
@@ -58,6 +62,9 @@ in
       RemainAfterExit = true;
       ExecStart = "${bootstrap-script}/bin/nixos-bootstrap";
 
+      # Read environment variables from /etc/environment (set by cloud-init)
+      EnvironmentFile = "-/etc/environment";
+
       # Logging to journald
       StandardOutput = "journal+console";
       StandardError = "journal+console";