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/create.yml

@@ -1,24 +1,27 @@
 ---
-- name: Create an LXC container
-  community.proxmox.proxmox:
-    vmid: "{{ lxc_vmid | default(omit) }}"
-    hostname: "{{ lxc_hostname }}"
-    password: "{{ lxc_root_password | default(omit) }}"
-    ostemplate: "{{ lxc_template }}"
-    cores: "{{ lxc_cores }}"
-    memory: "{{ lxc_memory }}"
-    swap: "{{ lxc_swap }}"
-    disk: "{{ lxc_disk }}"
-    mounts: "{{ lxc_mounts | default(omit) }}"
-    netif: >-
-      {"net0": "name={{ lxc_iface_name }},bridge={{ lxc_bridge }},ip={{ lxc_ipv4 }},gw={{ lxc_gateway }},ip6={{ lxc_ipv6 }}{% if lxc_vlan_tag is defined %},tag={{ lxc_vlan_tag }}{% endif %}"}
-    pubkey: "{{ lookup('file', lxc_pubkey_file) | default(omit) }}"
-    onboot: "{{ lxc_onboot | default(false) }}"
-    startup: "{{ lxc_startup | default(omit) }}"
-    unprivileged: "{{ lxc_unprivileged | default(true) }}"
-    features: "{{ lxc_features | default(omit) }}"
-    timezone: "{{ lxc_timezone | default(omit) }}"
-    nameserver: "{{ lxc_nameserver | default(omit) }}"
-    state: present
-    tags: "{{ lxc_tags | default(omit) }}"
-  register: lxc_result
+- name: Create LXC container
+  module_defaults: "{{ _proxmox_module_defaults }}"
+  block:
+    - name: Create an LXC container
+      community.proxmox.proxmox:
+        vmid: "{{ lxc_vmid | default(omit) }}"
+        hostname: "{{ lxc_hostname }}"
+        password: "{{ lxc_root_password | default(omit) }}"
+        ostemplate: "{{ lxc_template }}"
+        cores: "{{ lxc_cores }}"
+        memory: "{{ lxc_memory }}"
+        swap: "{{ lxc_swap }}"
+        disk: "{{ lxc_disk }}"
+        mounts: "{{ lxc_mounts | default(omit) }}"
+        netif: >-
+          {"net0": "name={{ lxc_iface_name }},bridge={{ lxc_bridge }},ip={{ lxc_ipv4 }},gw={{ lxc_gateway }},ip6={{ lxc_ipv6 }}{% if lxc_vlan_tag is defined %},tag={{ lxc_vlan_tag }}{% endif %}"}
+        pubkey: "{{ lookup('file', lxc_pubkey_file) | default(omit) }}"
+        onboot: "{{ lxc_onboot | default(false) }}"
+        startup: "{{ lxc_startup | default(omit) }}"
+        unprivileged: "{{ lxc_unprivileged | default(true) }}"
+        features: "{{ lxc_features | default(omit) }}"
+        timezone: "{{ lxc_timezone | default(omit) }}"
+        nameserver: "{{ lxc_nameserver | default(omit) }}"
+        state: present
+        tags: "{{ lxc_tags | default(omit) }}"
+      register: lxc_result