fix: k3s-agent label/taint tasks use undefined ansible_hostname

ansible.cfg sets inject_facts_as_vars: False repo-wide (the ollama role
already follows this via ansible_facts['system']), but these two tasks
referenced the bare ansible_hostname magic var, which doesn't exist under
that setting. Surfaced today as a hard failure re-running setup-gpu-node -
previously latent since these nodes' labels/taints were already correct
from whenever they last applied cleanly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ansible/roles/k3s-agent/tasks/main.yaml

@@ -24,7 +24,7 @@
 - name: Apply node labels
   ansible.builtin.shell:
     cmd: >
-      k3s kubectl label node {{ ansible_hostname }}
+      k3s kubectl label node {{ ansible_facts['hostname'] }}
       {{ item.key }}={{ item.value }} --overwrite
   loop: "{{ k3s_node_labels | dict2items }}"
   delegate_to: minisforum
@@ -35,7 +35,7 @@
 - name: Apply node taints
   ansible.builtin.shell:
     cmd: >
-      k3s kubectl taint node {{ ansible_hostname }}
+      k3s kubectl taint node {{ ansible_facts['hostname'] }}
       {{ item }} --overwrite
   loop: "{{ k3s_node_taints }}"
   delegate_to: minisforum