chore(infra): simplify Terraform config and update docs for auto-install ISO

- Remove ISO upload and answer-file resources (pre-provisioned manually)
- Remove null_resource wait/token steps (token created out-of-band)
- Switch iso_file to iso_file_id referencing an existing Proxmox file ID
- Add lifecycle ignore_changes for started and cdrom
- Update README with proxmox-auto-install-assistant ISO prep instructions
- Update default variables to match homelab environment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/infrastructure/.gitignore

@@ -4,3 +4,4 @@
 !terraform.tfvars.example
 .terraform.lock.hcl
 .api-token
+.build/