fix: [nomad-step-2] S2.3 review round 1 — document new helper + script, drop unused vault CLI precondition (#881)

Review feedback from PR #895 round 1:

- lib/AGENTS.md (hvault.sh row): add hvault_get_or_empty(PATH) to the
  public-function list; replace the "not sourced at runtime yet" note
  with the three actual callers (vault-apply-policies.sh,
  vault-apply-roles.sh, vault-nomad-auth.sh).
- lib/AGENTS.md (lib/init/nomad/ row): add a one-line description of
  vault-nomad-auth.sh (Step 2, this PR); relabel the row header from
  "Step 0 installer scripts" to "installer scripts" since it now spans
  Step 0 + Step 2.
- lib/init/nomad/vault-nomad-auth.sh: drop the `vault` CLI from the
  binary precondition check — hvault.sh's helpers are all curl-based,
  so the CLI is never invoked. The precondition would spuriously die on
  a Nomad-client-only node that has Vault server reachable but no
  `vault` binary installed. Inline comment preserves the rationale.

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

lib/init/nomad/vault-nomad-auth.sh

@@ -63,7 +63,11 @@ if [ "$(id -u)" -ne 0 ]; then
   die "must run as root (writes ${SERVER_HCL_DST} + signals nomad)"
 fi
 
-for bin in curl jq vault systemctl; do
+# curl + jq are used directly; hvault.sh's helpers are also curl-based, so
+# the `vault` CLI is NOT required here — don't add it to this list, or a
+# Vault-server-present / vault-CLI-absent box (e.g. a Nomad-client-only
+# node) would die spuriously. systemctl is required for SIGHUPing nomad.
+for bin in curl jq systemctl; do
   command -v "$bin" >/dev/null 2>&1 \
     || die "required binary not found: ${bin}"
 done