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/tasks/delete.yml

@@ -1,12 +1,15 @@
 ---
-- ansible.builtin.include_tasks: stop.yml
+- name: Delete LXC container
+  module_defaults: "{{ _proxmox_module_defaults }}"
+  block:
+    - ansible.builtin.include_tasks: stop.yml
 
-- name: Delete a container
-  community.proxmox.proxmox:
-    vmid: "{{ lxc_vmid | default(omit) }}"
-    hostname: "{{ lxc_hostname | default(omit) }}"
-    state: absent
-  register: delete_result
-  failed_when: |
-    delete_result.failed and
-    ('does not exist' not in delete_result.msg)
+    - name: Delete a container
+      community.proxmox.proxmox:
+        vmid: "{{ lxc_vmid | default(omit) }}"
+        hostname: "{{ lxc_hostname | default(omit) }}"
+        state: absent
+      register: delete_result
+      failed_when: |
+        delete_result.failed and
+        ('does not exist' not in delete_result.msg)