terraform: refactor cloud-init to use proxmox_cloud_init_disk resource

Replace SSH upload approach with native proxmox_cloud_init_disk resource
for cleaner, more maintainable cloud-init management.

Changes:
- Use proxmox_cloud_init_disk for all VMs (not just branch-specific ones)
- Include SSH keys, network config, and metadata in cloud-init disk
- Conditionally include NIXOS_FLAKE_BRANCH for VMs with flake_branch set
- Replace ide2 cloudinit disk with cdrom reference to cloud-init disk
- Remove built-in cloud-init parameters (ciuser, sshkeys, etc.)
- Remove cicustom parameter (no longer needed)
- Remove proxmox_host variable (no SSH uploads required)
- Remove .gitignore entry for .generated/ directory

Benefits:
- No SSH access to Proxmox required
- All cloud-init config managed in Terraform
- Consistent approach for all VMs
- Cleaner state management

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

.gitignore

@@ -10,4 +10,3 @@ terraform/terraform.tfvars
 terraform/*.auto.tfvars
 terraform/crash.log
 terraform/crash.*.log
-terraform/.generated/