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>

nomad/client.hcl

@@ -49,6 +49,12 @@ client {
     read_only = false
   }
 
+  # staging static content (docker/ directory with images, HTML, etc.)
+  host_volume "site-content" {
+    path      = "/srv/disinto/docker"
+    read_only = true
+  }
+
   # disinto chat transcripts + attachments.
   host_volume "chat-history" {
     path      = "/srv/disinto/chat-history"