Add logarchive gpg key + logging_logarchiver read access

logarchiver encrypts archived logs to an OpenPGP key held in the gpg engine so
the private key never leaves Vault; retrieval delegates decryption back to
gpg/decrypt/logarchive. This provisions the key and lets the service read only
its public key via a dedicated k8s auth role.

- Create gpg key logarchive (rsa-4096, non-exportable) in the gpg mount.
- Add k8s auth role logging_logarchiver bound to the logarchiver ServiceAccount
  in the logging namespace.
- Add policy granting read on gpg/keys/logarchive to that role (public key only;
  no decrypt/export).

Claude-Session: https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv

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

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

policies/gpg/logarchive_public_read.yaml

@@ -0,0 +1,12 @@
+# Allow the logarchiver service (logging namespace, SA logarchiver) to read the
+# logarchive public key. A plain read on gpg/keys/logarchive returns the armored
+# public_key; no decrypt/export capability is granted (decrypt stays operator-only).
+---
+rules:
+  - path: "gpg/keys/logarchive"
+    capabilities:
+      - read
+
+auth:
+  k8s/au/syd1:
+    - logging_logarchiver