refactor: use module_defaults for proxmox API connection

- Replace YAML merge keys (<<:) with module_defaults block in main.yml
- Simplify connection variables to individual vars instead of dictionary
- Remove redundant connection params from individual task files
- Document standalone task usage requires play-level module_defaults
- Update README examples with new variable pattern

roles/proxmox_lxc_provision/tasks/check-exists.yml

@@ -1,7 +1,6 @@
 ---
 - name: Query Proxmox for existing LXCs
-  community.proxmox.proxmox_lxc_info:
-    <<: "{{ proxmox_api_connection }}"
+  community.proxmox.proxmox_lxc_info: {}
   register: proxmox_lxcs
 
 - name: Check if LXC with hostname already exists