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/lib-systemd.sh

@@ -0,0 +1,70 @@
+#!/usr/bin/env bash
+# =============================================================================
+# lib/init/nomad/lib-systemd.sh — Shared idempotent systemd-unit installer
+#
+# Sourced by lib/init/nomad/systemd-nomad.sh and lib/init/nomad/systemd-vault.sh
+# (and any future sibling) to collapse the "write unit if content differs,
+# daemon-reload, enable (never start)" boilerplate.
+#
+# Install-but-don't-start is the invariant this helper enforces — mid-migration
+# installers land files and enable units; the orchestrator (S0.4) starts them.
+#
+# Public API (sourced into caller scope):
+#
+#   systemd_require_preconditions UNIT_PATH
+#     Asserts the caller is uid 0 and `systemctl` is on $PATH. Calls the
+#     caller's die() with a UNIT_PATH-scoped message on failure.
+#
+#   systemd_install_unit UNIT_PATH UNIT_NAME UNIT_CONTENT
+#     Writes UNIT_CONTENT to UNIT_PATH (0644 root:root) only if on-disk
+#     content differs. If written, runs `systemctl daemon-reload`. Then
+#     enables UNIT_NAME (no-op if already enabled). Never starts the unit.
+#
+# Caller contract:
+#   - Callers MUST define `log()` and `die()` before sourcing this file (we
+#     call log() for status chatter and rely on the caller's error-handling
+#     stance; `set -e` propagates install/cmp/systemctl failures).
+# =============================================================================
+
+# systemd_require_preconditions UNIT_PATH
+systemd_require_preconditions() {
+  local unit_path="$1"
+  if [ "$(id -u)" -ne 0 ]; then
+    die "must run as root (needs write access to ${unit_path})"
+  fi
+  command -v systemctl >/dev/null 2>&1 \
+    || die "systemctl not found (systemd is required)"
+}
+
+# systemd_install_unit UNIT_PATH UNIT_NAME UNIT_CONTENT
+systemd_install_unit() {
+  local unit_path="$1"
+  local unit_name="$2"
+  local unit_content="$3"
+
+  local needs_reload=0
+  if [ ! -f "$unit_path" ] \
+     || ! printf '%s\n' "$unit_content" | cmp -s - "$unit_path"; then
+    log "writing unit → ${unit_path}"
+    local tmp
+    tmp="$(mktemp)"
+    printf '%s\n' "$unit_content" > "$tmp"
+    install -m 0644 -o root -g root "$tmp" "$unit_path"
+    rm -f "$tmp"
+    needs_reload=1
+  else
+    log "unit file already up to date"
+  fi
+
+  if [ "$needs_reload" -eq 1 ]; then
+    log "systemctl daemon-reload"
+    systemctl daemon-reload
+  fi
+
+  if systemctl is-enabled --quiet "$unit_name" 2>/dev/null; then
+    log "${unit_name} already enabled"
+  else
+    log "systemctl enable ${unit_name}"
+    systemctl enable "$unit_name" >/dev/null
+  fi
+}