fix: [nomad-step-0] S0.3 — install vault + systemd auto-unseal + vault-init.sh (dev-persisted seal) (#823)

Adds the Vault half of the factory-dev-box bringup, landed but not started
(per the install-but-don't-start pattern used for nomad in #822):

- lib/init/nomad/install.sh — now also installs vault from the shared
  HashiCorp apt repo. VAULT_VERSION pinned (1.18.5). Fast-path skips apt
  entirely when both binaries are at their pins; partial upgrades only
  touch the package that drifted.

- nomad/vault.hcl — single-node config: file storage backend at
  /var/lib/vault/data, localhost listener on :8200, ui on, mlock kept on.
  No TLS / HA / audit yet; those land in later steps.

- lib/init/nomad/systemd-vault.sh — writes /etc/systemd/system/vault.service
  (Type=notify, ExecStartPost auto-unseals from /etc/vault.d/unseal.key,
  CAP_IPC_LOCK granted for mlock), deploys nomad/vault.hcl to
  /etc/vault.d/, creates /var/lib/vault/data (0700 root), enables the
  unit without starting it. Idempotent via content-compare.

- lib/init/nomad/vault-init.sh — first-run init: spawns a temporary
  `vault server` if not already reachable, runs operator-init with
  key-shares=1/threshold=1, persists unseal.key + root.token (0400 root),
  unseals once in-process, shuts down the temp server. Re-run detects
  initialized + unseal.key present → no-op. Initialized but key missing
  is a hard failure (can't recover).

lib/hvault.sh already defaults VAULT_TOKEN to /etc/vault.d/root.token
when the env var is absent, so no change needed there.

Seal model: the single unseal key lives on disk; seal-key theft equals
vault theft. Factory-dev-box-acceptable tradeoff — avoids running a
second Vault to auto-unseal the first.

Blocks S0.4 (#824).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/init/nomad/systemd-nomad.sh

@@ -33,13 +33,11 @@ NOMAD_DATA_DIR="/var/lib/nomad"
 log() { printf '[systemd-nomad] %s\n' "$*"; }
 die() { printf '[systemd-nomad] ERROR: %s\n' "$*" >&2; exit 1; }
 
-# ── Preconditions ────────────────────────────────────────────────────────────
-if [ "$(id -u)" -ne 0 ]; then
-  die "must run as root (needs write access to ${UNIT_PATH})"
-fi
+# shellcheck source=lib-systemd.sh
+. "$(dirname "${BASH_SOURCE[0]}")/lib-systemd.sh"
 
-command -v systemctl >/dev/null 2>&1 \
-  || die "systemctl not found (systemd is required)"
+# ── Preconditions ────────────────────────────────────────────────────────────
+systemd_require_preconditions "$UNIT_PATH"
 
 NOMAD_BIN="$(command -v nomad 2>/dev/null || true)"
 [ -n "$NOMAD_BIN" ] \
@@ -98,33 +96,7 @@ for d in "$NOMAD_CONFIG_DIR" "$NOMAD_DATA_DIR"; do
   fi
 done
 
-# ── Install unit file only if content differs ────────────────────────────────
-needs_reload=0
-if [ ! -f "$UNIT_PATH" ] \
-   || ! printf '%s\n' "$DESIRED_UNIT" | cmp -s - "$UNIT_PATH"; then
-  log "writing unit → ${UNIT_PATH}"
-  tmp="$(mktemp)"
-  trap 'rm -f "$tmp"' EXIT
-  printf '%s\n' "$DESIRED_UNIT" > "$tmp"
-  install -m 0644 -o root -g root "$tmp" "$UNIT_PATH"
-  rm -f "$tmp"
-  trap - EXIT
-  needs_reload=1
-else
-  log "unit file already up to date"
-fi
-
-# ── Reload + enable ──────────────────────────────────────────────────────────
-if [ "$needs_reload" -eq 1 ]; then
-  log "systemctl daemon-reload"
-  systemctl daemon-reload
-fi
-
-if systemctl is-enabled --quiet nomad.service 2>/dev/null; then
-  log "nomad.service already enabled"
-else
-  log "systemctl enable nomad"
-  systemctl enable nomad.service >/dev/null
-fi
+# ── Install + reload + enable (shared with systemd-vault.sh via lib-systemd) ─
+systemd_install_unit "$UNIT_PATH" "nomad.service" "$DESIRED_UNIT"
 
 log "done — unit installed and enabled (NOT started; S0.4 brings the cluster up)"