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>

tests/disinto-init-nomad.bats

@@ -215,7 +215,7 @@ setup_file() {
   run "$DISINTO_BIN" init placeholder/repo --backend=nomad --with unknown-service --dry-run
   [ "$status" -ne 0 ]
   [[ "$output" == *"unknown service"* ]]
-  [[ "$output" == *"known: forgejo, woodpecker-server, woodpecker-agent, agents"* ]]
+  [[ "$output" == *"known: forgejo, woodpecker-server, woodpecker-agent, agents, staging, chat"* ]]
 }
 
 # S3.4: woodpecker auto-expansion and forgejo auto-inclusion