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/start.yml

@@ -1,7 +1,6 @@
 ---
 - name: Start the LXC container
   community.general.proxmox:
-    <<: "{{ proxmox_api_connection }}"
     vmid: "{{ lxc_result.vmid }}"
     state: started
   register: start_result