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]>

ci/yamllint.yaml

@@ -7,6 +7,10 @@ yaml-files:
   - "*.yaml"
   - "*.yml"
 
+# ansible-vault-encrypted files are a `$ANSIBLE_VAULT;...` blob, not parseable YAML — skip them.
+ignore: |
+  **/vault.yml
+
 rules:
   # Kubernetes / Kustomize / Helm manifests routinely exceed 80 cols; allow room
   # but keep a sane ceiling so lines stay reviewable.