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/defaults/main.yml

@@ -1,4 +1,9 @@
 ---
+# Proxmox API connection defaults (optional)
+proxmox_api_port: 8006
+proxmox_api_validate_certs: false
+
+# LXC defaults
 lxc_template: "local:vztmpl/debian-12-standard_12.12-1_amd64.tar.zst"
 lxc_cores: 4
 lxc_memory: 2048