policies: let terraform-git seed the gitea engine admin credential to KV (#102)

## Why

terraform-git now provisions the `gitea-vault-admin` site-admin bot and writes its generated password to `kv/service/vault/au/syd1/secret_backend/gitea/config` (as `admin_username` + `admin_password`) so the gitea secrets engine can consume it at creation time. The `woodpecker_terraform_git` / `terraform_git` identity has no write access to that KV path, so its apply would 403 without this grant.

The deployer that *reads* the seed already has read access via `policies/kv/service/vault/secret_backends_read.yaml` (`kv/data/service/vault/+/+/secret_backend/*`), so only the write side is added here.

## Change

- Add `policies/kv/service/vault/au/syd1/secret_backend/gitea/config_write.yaml` granting `create`/`read`/`update` on the gitea config KV path to the `terraform_git` approle and `woodpecker_terraform_git` k8s role.

## Ordering

Merge + apply this before the terraform-git `benvin/gitea-vault-admin` PR applies (which performs the write). Files are disjoint from the other gitea terraform-vault PRs (#100, #101).

https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv
Reviewed-on: #102
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

policies/kv/service/vault/au/syd1/secret_backend/gitea/config_write.yaml

@@ -0,0 +1,20 @@
+# Allow terraform-git to seed (write once) the gitea secrets engine's admin
+# credentials. terraform-git creates the gitea-vault-admin site-admin bot and
+# writes its generated password here as admin_username + admin_password; the
+# vault gitea engine (managed by the tf_vault deployer) reads it at gitea/config
+# creation time. Read is already granted to the deployer via
+# policies/kv/service/vault/secret_backends_read.yaml, so this only adds the
+# write side for terraform-git's own identity.
+---
+rules:
+  - path: "kv/data/service/vault/au/syd1/secret_backend/gitea/config"
+    capabilities:
+      - create
+      - read
+      - update
+
+auth:
+  approle:
+    - terraform_git
+  k8s/au/syd1:
+    - woodpecker_terraform_git