Scaffold vault-plugin-secrets-arrstack engine

Mint dynamic arrproxy machine tokens via arrproxy's bearer-gated admin API
so Terraform-driven *arr onboarding can issue and revoke per-role tokens
non-interactively.

- Add backend, config, roles, creds paths and the arrstack_token secret
- Call POST/DELETE /api/admin/tokens with a vault:arrstack:<role> subject
- Enforce apps as a non-empty subset of sonarr/radarr/prowlarr
- Cap lease renewal at the arrproxy token's fixed expiry
- Add table-driven unit tests against a fake arrproxy admin server
- Add Makefile, nfpm packaging, and pre-commit/build/test/release pipelines

.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