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/ansible.cfg

@@ -7,6 +7,10 @@ retry_files_enabled = False
 stdout_callback = default
 result_format = yaml
 interpreter_python = auto_silent
+# Vault passphrase file — kept OUTSIDE the repo (never committed). Decrypts the vaulted
+# group_vars (e.g. the host become password) so plays run unattended. Create it once:
+#   install -m600 /dev/stdin ~/.config/home-k3s/ansible-vault-pass <<<'YOUR_VAULT_PASSPHRASE'
+vault_password_file = ~/.config/home-k3s/ansible-vault-pass
 
 [ssh_connection]
 pipelining = True