fix: [nomad-step-5] S5.2 — nomad/jobs/staging.hcl + chat.hcl (#989)

Add lightweight Nomad service jobs for the staging file server and
Claude chat UI. Key changes:

- nomad/jobs/staging.hcl: caddy:alpine file-server mounting docker/
  as /srv/site (read-only), no Vault integration needed
- nomad/jobs/chat.hcl: custom disinto/chat:local image with sandbox
  hardening (cap_drop ALL, tmpfs, pids_limit 128, security_opt),
  Vault-templated OAuth secrets from kv/disinto/shared/chat
- nomad/client.hcl: add site-content host volume for staging
- vault/policies/service-chat.hcl + vault/roles.yaml: read-only
  access to chat secrets via workload identity
- bin/disinto: wire staging+chat into build, deploy order, seed
  mapping, summary, and service validation
- tests/disinto-init-nomad.bats: update known-services assertion

Fixes prior art issue where security_opt and pids_limit were placed
at task level instead of inside docker driver config block.

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

vault/policies/service-chat.hcl

@@ -0,0 +1,15 @@
+# vault/policies/service-chat.hcl
+#
+# Read-only access to shared Chat secrets (OAuth client config, forward auth
+# secret). Attached to the Chat Nomad job via workload identity (S5.2).
+#
+# Scope: kv/disinto/shared/chat — entries owned by the operator and
+# shared between the chat service and edge proxy.
+
+path "kv/data/disinto/shared/chat" {
+  capabilities = ["read"]
+}
+
+path "kv/metadata/disinto/shared/chat" {
+  capabilities = ["list", "read"]
+}