feat(vault): deploy HashiCorp Vault 2.0.1 via Helm chart (5-replica HA raft) (#148)

## Summary

- Deploys HashiCorp Vault 2.0.1 using Helm chart 0.32.0 in HA raft mode (5 replicas)
- Configuration modelled on production vault: \`disable_mlock=true\`, headless-DNS retry_join for all 5 pods
- IPC_LOCK capability added via \`server.statefulSet.securityContext.container\`
- 10Gi cephrbd-fast-delete PVC per pod via \`dataStorage\`
- Gateway API: HTTPS gateway + HTTPRoute (443→vault service port 8200) at \`vault.k8s.syd1.au.unkin.net\`
- ArgoCD platform ApplicationSet updated to include vault overlay path
- Injector disabled (no agent sidecar injection needed)

## Requires

- PR #147 (artifactapi: add hashicorp/vault to docker immutable patterns) to be merged first

## Test plan

- [ ] Sandbox tested in \`sandbox-vault\`: all 5 pods Running, raft cluster forming
- [ ] After merge: ArgoCD syncs vault namespace
- [ ] Operator runs \`vault operator init\` to initialize, then unseals all 5 nodes
- [ ] Verify \`vault.k8s.syd1.au.unkin.net\` is accessible via Gateway

Reviewed-on: #148

argocd/applicationsets/platform.yaml

@@ -27,6 +27,7 @@ spec:
       - path: apps/overlays/*/reposync
       - path: apps/overlays/*/traefik-system
       - path: apps/overlays/*/vm-system
+      - path: apps/overlays/*/vault
       - path: apps/overlays/*/vso-system
       - path: apps/overlays/*/woodpecker
   template: