fix: use direct in-cluster kubernetes auth instead of kubeconfig file

- providers.tf: use host + token + ca_crt from mounted service account secrets
- workflow: remove kubeconfig generation step (no longer needed)
- variables.tf: remove unused kubeconfig_path variable

This is the standard pattern for running terraform inside k8s pods.

terraform/variables.tf

@@ -1,9 +1,3 @@
-variable "kubeconfig_path" {
-  description = "Path to kubeconfig file"
-  type        = string
-  default     = "/tmp/kubeconfig"
-}
-
 variable "cluster_domain" {
   description = "Cluster domain (e.g., riotpiao.homelab.com)"
   type        = string