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/group_vars/hypervisor/README.md

@@ -0,0 +1,37 @@
+# group_vars for the `hypervisor` host (the workstation)
+
+The host-side plays (`playbooks/host-prereqs.yml`) run against the `hypervisor` group and need
+**become (sudo) on the host**. Supply the password as an **ansible-vault-encrypted** var here — it is
+encrypted, so it is safe to commit; the passphrase that decrypts it stays out of the repo.
+
+## Two files (one committed-encrypted, one never committed)
+
+1. **The vault passphrase** — a file **outside the repo**, referenced by `ansible.cfg`
+   (`vault_password_file = ~/.config/home-k3s/ansible-vault-pass`). Create it once:
+   ```bash
+   mkdir -p ~/.config/home-k3s
+   install -m600 /dev/stdin ~/.config/home-k3s/ansible-vault-pass <<<'YOUR_VAULT_PASSPHRASE'
+   ```
+
+2. **The encrypted become password** — `vault.yml` in **this directory** (committed, encrypted):
+   ```bash
+   cd infra/ansible
+   ansible-vault create inventory/group_vars/hypervisor/vault.yml
+   # file content (single line):
+   #   ansible_become_password: "YOUR_HOST_SUDO_PASSWORD"
+   ```
+   (`ansible-vault create` uses the passphrase file above automatically — no prompt.)
+
+Then a rebuild is unattended:
+```bash
+cd infra/ansible && ansible-playbook playbooks/host-prereqs.yml
+```
+
+## Notes
+- `vault.yml` is skipped by the yamllint gate (`ci/yamllint.yaml` ignores `**/vault.yml`) because a
+  vault blob is not parseable YAML.
+- The k3s **nodes** use passwordless sudo (`group_vars/all.yml`), so they need no become password —
+  this vault is scoped to the `hypervisor` host only.
+- If your host already has passwordless sudo, you can skip the vault and just run the playbook.
+- This is the Ansible-bootstrap-layer secret (host credential), distinct from the platform's
+  in-cluster **SOPS + age** secrets (ADR-0007) — different layer, different tool.

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