refactor(proxmox_lxc_provision): centralize module_defaults so tasks_from works without setup

Previously the community.proxmox.proxmox / proxmox_vm_info module_defaults
were defined inline on the outer block in main.yml. Invoking individual
task files via 'tasks_from: stop' (or delete/convert/etc.) bypassed
main.yml, leaving the API parameters unset and producing
'missing required arguments: api_host, api_user' errors. The README
worked around this by telling callers to repeat the module_defaults
block at the play level — easy to forget, and duplicated config.

Extract the defaults dict into _proxmox_module_defaults in
defaults/main.yml (using a YAML anchor to share between the two
modules), and wrap every task file that calls a Proxmox module in a
block that references it. Callers only need the proxmox_* connection
vars in scope (typically group_vars/all/) — both 'roles:' and
'tasks_from:' invocations now configure the API consistently.

Files wrapped: check-exists, create, clone, update, start, stop,
delete, convert. wait/post-clone/edit-config don't call Proxmox modules
and are unchanged. main.yml's now-redundant outer module_defaults is
removed.

README updated to drop the 'Using Standalone Tasks' workaround
boilerplate.

roles/proxmox_lxc_provision/README.md

@@ -144,26 +144,17 @@ The role includes idempotency checking. If a container with the specified `lxc_v
 
 ### Using Standalone Tasks
 
-When using individual task files via `tasks_from`, you must set `module_defaults` at the play level since the tasks bypass the role's main entry point:
+Individual task files (`stop`, `start`, `delete`, `convert`, etc.) can be invoked via `tasks_from` directly — each task file wraps its work in a block with the role's shared `module_defaults`, so the Proxmox API connection is configured automatically as long as the `proxmox_*` connection variables are in scope (typically from `group_vars/all/`).
 
 ```yaml
 - name: Convert container to a template
   hosts: localhost
-  module_defaults:
-    community.proxmox.proxmox:
-      api_host: "{{ proxmox_api_host }}"
-      api_port: "{{ proxmox_api_port }}"
-      api_user: "{{ proxmox_api_user }}"
-      api_token_id: "{{ proxmox_api_token_id }}"
-      api_token_secret: "{{ proxmox_api_token_secret }}"
-      validate_certs: "{{ proxmox_api_validate_certs }}"
-      node: "{{ proxmox_node }}"
-  vars:
-    lxc_hostname: "{{ lxc_hostname }}"
   tasks:
     - include_role:
         name: proxmox_lxc_provision
         tasks_from: convert
+      vars:
+        lxc_hostname: my-container
 ```
 
 ### Creating an LXC Container and Converting it to a Template
@@ -185,19 +176,10 @@ When using individual task files via `tasks_from`, you must set `module_defaults
 
 - name: Convert the created container to a template
   hosts: localhost
-  module_defaults:
-    community.proxmox.proxmox:
-      api_host: "{{ proxmox_api_host }}"
-      api_port: "{{ proxmox_api_port }}"
-      api_user: "{{ proxmox_api_user }}"
-      api_token_id: "{{ proxmox_api_token_id }}"
-      api_token_secret: "{{ proxmox_api_token_secret }}"
-      validate_certs: "{{ proxmox_api_validate_certs }}"
-      node: "{{ proxmox_node }}"
-  vars:
-    lxc_hostname: "{{ lxc_hostname }}"
   tasks:
     - include_role:
         name: proxmox_lxc_provision
         tasks_from: convert
+      vars:
+        lxc_hostname: "{{ lxc_hostname }}"
 ```