OS upgrade complete: Debian 12->13 (trixie) baseline + re-provision role fixes

All 3 k3s nodes re-provisioned to Debian 13 (kernel 6.12) via immutable tofu -replace,
one at a time, leader/primary last. variables.tf image defaults -> trixie generic.
k3s_server role: ensure /etc/rancher/k3s exists (fresh-node re-provision fix).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

infra/tofu/variables.tf

@@ -182,7 +182,7 @@ variable "image_source" {
     import + disk-resize under bpg (bpg issue #1639).
   EOT
   type        = string
-  default     = "https://cloud.debian.org/images/cloud/bookworm/latest/debian-12-generic-amd64.qcow2"
+  default     = "https://cloud.debian.org/images/cloud/trixie/latest/debian-13-generic-amd64.qcow2"
 }
 
 variable "image_file_name" {
@@ -194,7 +194,7 @@ variable "image_file_name" {
     panics with "Attempted to kill init" on import + disk-resize (bpg issue #1639).
   EOT
   type        = string
-  default     = "debian-12-generic-amd64.qcow2"
+  default     = "debian-13-generic-amd64.qcow2"
 }
 
 # --- cloud-init identity ----------------------------------------------------