fix: [nomad-step-2] S2.3 — vault-nomad-auth.sh (enable JWT auth + roles + nomad workload identity) (#881)

Wires Nomad → Vault via workload identity so jobs can exchange their
short-lived JWT for a Vault token carrying the policies in
vault/policies/ — no shared VAULT_TOKEN in job env.

- `lib/init/nomad/vault-nomad-auth.sh` — idempotent script: enable jwt
  auth at path `jwt-nomad`, config JWKS/algs, apply roles, install
  server.hcl + SIGHUP nomad on change.
- `tools/vault-apply-roles.sh` — companion sync script (S2.1 sibling);
  reads vault/roles.yaml and upserts each Vault role under
  auth/jwt-nomad/role/<name> with created/updated/unchanged semantics.
- `vault/roles.yaml` — declarative role→policy→bound_claims map; one
  entry per vault/policies/*.hcl. Keeps S2.1 policies and S2.3 role
  bindings visible side-by-side at review time.
- `nomad/server.hcl` — adds vault stanza (enabled, address,
  default_identity.aud=["vault.io"], ttl=1h).
- `lib/hvault.sh` — new `hvault_get_or_empty` helper shared between
  vault-apply-policies.sh, vault-apply-roles.sh, and vault-nomad-auth.sh;
  reads a Vault endpoint and distinguishes 200 / 404 / other.
- `vault/policies/AGENTS.md` — extends S2.1 docs with JWT-auth role
  naming convention, token shape, and the "add new service" flow.

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

nomad/server.hcl

@@ -51,3 +51,26 @@ advertise {
 ui {
   enabled = true
 }
+
+# ─── Vault integration (S2.3, issue #881) ───────────────────────────────────
+# Nomad jobs exchange their short-lived workload-identity JWT (signed by
+# nomad's built-in signer at /.well-known/jwks.json on :4646) for a Vault
+# token carrying the policies named by the role in `vault { role = "..." }`
+# of each jobspec — no shared VAULT_TOKEN in job env.
+#
+# The JWT auth path (jwt-nomad) + per-role bindings live on the Vault
+# side, written by lib/init/nomad/vault-nomad-auth.sh + tools/vault-apply-roles.sh.
+# Roles are defined in vault/roles.yaml.
+#
+# `default_identity.aud = ["vault.io"]` matches bound_audiences on every
+# role in vault/roles.yaml — a drift here would silently break every job's
+# Vault token exchange at placement time.
+vault {
+  enabled = true
+  address = "http://127.0.0.1:8200"
+
+  default_identity {
+    aud = ["vault.io"]
+    ttl = "1h"
+  }
+}