Wire ansible-vault for host become password (unattended host-prereqs)

Where the sudo password goes, codified:
- inventory: new [hypervisor] group (localhost, connection=local).
- host-prereqs.yml targets 'hypervisor'; become password from vaulted
  group_vars/hypervisor/vault.yml (ansible_become_password) — user creates it
  encrypted (safe to commit); README documents the exact commands.
- ansible.cfg vault_password_file = ~/.config/home-k3s/ansible-vault-pass
  (passphrase kept OUTSIDE the repo, never committed).
- ci/yamllint.yaml ignores **/vault.yml (vault blob isn't parseable YAML).
Nodes keep NOPASSWD sudo (group_vars/all) — vault scoped to the host only.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

infra/ansible/inventory/hosts.ini

@@ -15,3 +15,8 @@ k3s-dev-3 ansible_host=10.73.0.13
 [k3s_servers:children]
 k3s_primary
 k3s_secondary
+
+# The hypervisor/workstation host itself (target of host-prereqs.yml). Its become
+# (sudo) password comes from the vaulted group_vars/hypervisor/vault.yml.
+[hypervisor]
+localhost ansible_connection=local