vault: add ghp k8s auth role + kv read policy

ghp (a GitHub proxy) reads its GitHub App credentials and encryption key from kv/kubernetes/ghp/*; it uses the postgres storage backend so it only needs read access to those kv secrets.

- add k8s/au/syd1 auth role ghp bound to ServiceAccount ghp in namespace ghp (ttl 600s, audience vault)
- add read-only kv policy granting read on kv/data/kubernetes/ghp/* and read+list on kv/metadata/kubernetes/ghp/*
- mirrors the artifactapi / logging_logarchiver per-app pattern (app-SA-bound k8s role + single kv read policy consumed by VSO)

config/auth_kubernetes_role/k8s/au/syd1/ghp.yaml

@@ -0,0 +1,7 @@
+bound_service_account_names:
+  - ghp
+bound_service_account_namespaces:
+  - ghp
+token_ttl: 600
+token_max_ttl: 600
+audience: vault

policies/kv/kubernetes/ghp/read.yaml

@@ -0,0 +1,17 @@
+# Allow ghp to read its GitHub App credentials and encryption key
+#
+# kv/kubernetes/ghp/github-app  (app_id/client_id/client_secret/private_key)
+# kv/kubernetes/ghp/app         (encryption_key)
+---
+rules:
+  - path: "kv/data/kubernetes/ghp/*"
+    capabilities:
+      - read
+  - path: "kv/metadata/kubernetes/ghp/*"
+    capabilities:
+      - read
+      - list
+
+auth:
+  k8s/au/syd1:
+    - ghp