Initial vault-plugin-secrets-rancher scaffold

Vault/OpenBao secrets engine managing Rancher API tokens via the public
tokens.ext.cattle.io API.

- config: Rancher connection (URL + TLS)
- service-accounts/<name>: seeded root tokens, auto-rotated before Rancher's
  TTL cap via a PeriodicFunc (default 45d rotation, 90d token TTL); the current
  token mints its own replacement. Manual /rotate endpoint too.
- roles/<name>: mint policy referencing a service account; cluster_name + TTL
  scoping (Rancher tokens inherit the seeding user's RBAC).
- creds/<role>: dynamic, lease-bound tokens deleted from Rancher on revoke.

Ports the bind-tsig Woodpecker RPM release, nfpm packaging, and a mock-Rancher
e2e (Vault + OpenBao). Unit tests cover the full lifecycle.

.pre-commit-config.yaml

@@ -0,0 +1,15 @@
+repos:
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v5.0.0
+    hooks:
+      - id: trailing-whitespace
+      - id: end-of-file-fixer
+      - id: check-yaml
+      - id: check-added-large-files
+
+  - repo: https://github.com/dnephin/pre-commit-golang
+    rev: v0.5.1
+    hooks:
+      - id: go-fmt
+      - id: go-vet
+      - id: go-mod-tidy