Fix stale sftp IP, ansible.cfg keys, drop dead shutdown playbook

- sftp IP 192.168.99.68 in CLAUDE.md and server README (DMZ move)
- ansible.cfg: ansible_python_interpreter is a host var, moved to
  group_vars/all.yml; [ssh_connections] -> [ssh_connection] so
  pipelining actually applies
- remove playbooks/shutdown.yml (targeted nonexistent k3s_cluster group)

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

ansible/playbooks/shutdown.yml

@@ -1,9 +0,0 @@
----
-- name: Shutdown k3s_cluster
-  hosts: k3s_cluster
-  gather_facts: true
-  tasks:
-    - name: Shutdown the nodes (and wait one 1 min)
-      become: true
-      community.general.shutdown:
-        delay: 60