Add agent-dns RBAC: static SA + ClusterRole + per-namespace RoleBindings (#332)

## Why
Vault's kubernetes secret engine will mint scoped tokens for a static \`agent-dns\` service account instead of generating cluster-wide RBAC, so agent DNS access is confined to exactly the bind namespaces. This is the GitOps half of the terraform-vault agent-dns role rework (PR unkin/terraform-vault#109). Ordering: this must sync before the Vault \`agent-dns\` creds are usable — Vault mints tokens for an SA that must already exist.

## How
- Add ServiceAccount \`agent-dns\` + ClusterRole \`agent-dns\` (definition only, no ClusterRoleBinding) in \`bind-system\`: full verbs on \`bind.unkin.net\` CRDs, get/list/watch pods/services/configmaps/events, get pods/log.
- Add RoleBinding \`agent-dns\` in each of \`bind-system\`, \`bind-internal\`, \`bind-external\`, \`externaldns\`, binding the SA to the ClusterRole in that namespace — confining all access (reads included) to those four namespaces.

Whitelist note: the platform AppProject already permits ClusterRole/ClusterRoleBinding and all four namespace destinations, so no project change is needed.

https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT
---------

Co-authored-by: Ben Vincent <neotheo@gmail.com>
Reviewed-on: #332
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

apps/base/bind-external/agent-dns-rolebinding.yaml

@@ -0,0 +1,16 @@
+---
+# Confines the agent-dns service account (in bind-system) to the agent-dns
+# ClusterRole within this namespace.
+apiVersion: rbac.authorization.k8s.io/v1
+kind: RoleBinding
+metadata:
+  name: agent-dns
+  namespace: bind-external
+subjects:
+  - kind: ServiceAccount
+    name: agent-dns
+    namespace: bind-system
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: agent-dns

apps/base/bind-external/kustomization.yaml

@@ -7,3 +7,4 @@ resources:
   - cluster.yaml
   - tsigkey.yaml
   - zones.yaml
+  - agent-dns-rolebinding.yaml

apps/base/bind-internal/agent-dns-rolebinding.yaml

@@ -0,0 +1,16 @@
+---
+# Confines the agent-dns service account (in bind-system) to the agent-dns
+# ClusterRole within this namespace.
+apiVersion: rbac.authorization.k8s.io/v1
+kind: RoleBinding
+metadata:
+  name: agent-dns
+  namespace: bind-internal
+subjects:
+  - kind: ServiceAccount
+    name: agent-dns
+    namespace: bind-system
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: agent-dns

apps/base/bind-internal/kustomization.yaml

@@ -8,3 +8,4 @@ resources:
   - resolvers
   - externaldns
   - tsig-api
+  - agent-dns-rolebinding.yaml

apps/base/bind-system/agent-dns-rbac.yaml

@@ -0,0 +1,38 @@
+---
+# Static service account that Vault's kubernetes secret engine mints scoped
+# tokens for (agent-dns role). RBAC is confined to the bind namespaces via the
+# per-namespace RoleBindings below, not a ClusterRoleBinding.
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: agent-dns
+  namespace: bind-system
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRole
+metadata:
+  name: agent-dns
+rules:
+  - apiGroups: ["bind.unkin.net"]
+    resources: ["*"]
+    verbs: ["*"]
+  - apiGroups: [""]
+    resources: ["pods", "services", "configmaps", "events"]
+    verbs: ["get", "list", "watch"]
+  - apiGroups: [""]
+    resources: ["pods/log"]
+    verbs: ["get"]
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: RoleBinding
+metadata:
+  name: agent-dns
+  namespace: bind-system
+subjects:
+  - kind: ServiceAccount
+    name: agent-dns
+    namespace: bind-system
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: agent-dns

apps/base/bind-system/kustomization.yaml

@@ -8,5 +8,6 @@ resources:
   # vendored here, so they never drift from the operator.
   - https://git.unkin.net/unkin/bind-operator/raw/tag/v0.2.6/config/crd/install.yaml
   - rbac.yaml
+  - agent-dns-rbac.yaml
   - deployment.yaml
   - vpa.yaml

apps/base/externaldns/agent-dns-rolebinding.yaml

@@ -0,0 +1,16 @@
+---
+# Confines the agent-dns service account (in bind-system) to the agent-dns
+# ClusterRole within this namespace.
+apiVersion: rbac.authorization.k8s.io/v1
+kind: RoleBinding
+metadata:
+  name: agent-dns
+  namespace: externaldns
+subjects:
+  - kind: ServiceAccount
+    name: agent-dns
+    namespace: bind-system
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: agent-dns

apps/base/externaldns/kustomization.yaml

@@ -6,3 +6,4 @@ resources:
   - namespace.yaml
   - vaultauth.yaml
   - vaultstaticsecret.yaml
+  - agent-dns-rolebinding.yaml