Add Vault-scoped agent kubernetes roles + agents AppRole (#109)

## Why
Agentic workloads currently need cluster-admin/root kubeconfig contexts to do routine per-domain work. This adds domain-scoped, Vault-issued kubernetes credentials plus an `agents` AppRole so agents get least-privilege access instead of escalating.

## Changes
- Add `kubernetes_secret_backend_role` configs `agent-dhcp`, `agent-dns`, `agent-certs`, `agent-storage` (au/syd1):
  - **agent-dhcp** (Role, ns `dhcp-system`): full verbs on `kea.unkin.net` CRDs; get/list/watch pods/services/configmaps/events + pods/log.
  - **agent-dns** (`service_account_name` mode): mints tokens for the static `agent-dns` SA (argocd-apps#332) whose per-namespace RoleBindings confine access to bind-system/bind-internal/bind-external/externaldns. `allowed_kubernetes_namespaces: [bind-system]` (the SA's namespace).
  - **agent-certs** (Role, ns `cert-manager`): full verbs on `cert-manager.io` + `acme.cert-manager.io` (closes the orders/challenges debugging gap); get/list/watch/delete secrets; get/list/watch pods + pods/log. Secret delete confined to `cert-manager`.
  - **agent-storage** (Role, ns `cephrgw-system`): full verbs on `ceph.unkin.net` CRDs (buckets/bucketaccesses/objectstoreusers); get/list/watch pods + pods/log.
- Extend the `kubernetes_secret_backend_role` module with an optional `service_account_name`; when set, `generated_role_rules`/`kubernetes_role_type` are omitted (the SA's own bindings supply RBAC).
- Add creds policies for each role, bound to the `kubernetes_au_syd1_cluster_operator` ldap group (human kubectl use) and the `agents` AppRole (programmatic use).
- Add the `agents` AppRole (mirrors the certmanager approle schema): `bind_secret_id: false` (role_id-only login), `token_bound_cidrs: [10.10.12.200/32]` (agent workstation wg0 addr), deterministic role_id, 1h/4h TTLs.
- Add `policies/kv/kubernetes/agents.yaml` granting the AppRole create/read/update/list on `kv/data/kubernetes/*` + read/list on `kv/metadata/kubernetes/*` (no delete).

## Ordering
argocd-apps#332 (the `agent-dns` SA + ClusterRole + per-namespace RoleBindings) must sync **before** the `agent-dns` creds here are usable — Vault mints tokens for a service account that must already exist.

https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT
Reviewed-on: #109
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

resources/secret_backend/kubernetes/au/syd1/roles/agent-certs.yaml

@@ -0,0 +1,33 @@
+---
+rules:
+  - apiGroups:
+      - "cert-manager.io"
+      - "acme.cert-manager.io"
+    resources:
+      - "*"
+    verbs:
+      - "*"
+  - apiGroups:
+      - ""
+    resources:
+      - "secrets"
+    verbs:
+      - "get"
+      - "list"
+      - "watch"
+      - "delete"
+  - apiGroups:
+      - ""
+    resources:
+      - "pods"
+    verbs:
+      - "get"
+      - "list"
+      - "watch"
+  - apiGroups:
+      - ""
+    resources:
+      - "pods/log"
+    verbs:
+      - "get"
+      - "list"

resources/secret_backend/kubernetes/au/syd1/roles/agent-dhcp.yaml

@@ -0,0 +1,26 @@
+---
+rules:
+  - apiGroups:
+      - "kea.unkin.net"
+    resources:
+      - "*"
+    verbs:
+      - "*"
+  - apiGroups:
+      - ""
+    resources:
+      - "pods"
+      - "services"
+      - "configmaps"
+      - "events"
+    verbs:
+      - "get"
+      - "list"
+      - "watch"
+  - apiGroups:
+      - ""
+    resources:
+      - "pods/log"
+    verbs:
+      - "get"
+      - "list"

resources/secret_backend/kubernetes/au/syd1/roles/agent-storage.yaml

@@ -0,0 +1,23 @@
+---
+rules:
+  - apiGroups:
+      - "ceph.unkin.net"
+    resources:
+      - "*"
+    verbs:
+      - "*"
+  - apiGroups:
+      - ""
+    resources:
+      - "pods"
+    verbs:
+      - "get"
+      - "list"
+      - "watch"
+  - apiGroups:
+      - ""
+    resources:
+      - "pods/log"
+    verbs:
+      - "get"
+      - "list"