feat: add bare bones Proxmox Terraform scaffold

Adds provider config, variables, and an example tfvars file using
the bpg/proxmox provider. No resources are managed yet — existing
Proxmox setup is untouched.

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

terraform.tfvars.example

@@ -0,0 +1,7 @@
+# Copy this file to terraform.tfvars and fill in your values.
+# terraform.tfvars is gitignored to keep secrets out of version control.
+
+proxmox_endpoint = "https://192.168.1.10:8006/"
+proxmox_username = "terraform@pve"
+proxmox_password = "changeme"
+proxmox_insecure = true