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

Makefile

@@ -0,0 +1,62 @@
+.PHONY: build install test lint fmt clean tidy rpm rpm-package patch minor major check-go
+
+BINARY := vault-plugin-secrets-arrstack
+PKG := ./cmd/vault-plugin-secrets-arrstack
+VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo "0.0.0-dev")
+OS ?= $(shell go env GOOS)
+ARCH ?= $(shell go env GOARCH)
+PLUGIN_DIR ?= ./dist
+
+GO_VERSION_REQUIRED := 1.25
+GO_VERSION_ACTUAL := $(shell go version | sed 's/go version go\([0-9]*\.[0-9]*\).*/\1/')
+
+check-go:
+	@if [ "$$(printf '%s\n%s' "$(GO_VERSION_REQUIRED)" "$(GO_VERSION_ACTUAL)" | sort -V | head -1)" != "$(GO_VERSION_REQUIRED)" ]; then \
+		echo "ERROR: Go >= $(GO_VERSION_REQUIRED) required, found $(GO_VERSION_ACTUAL)"; exit 1; \
+	fi
+
+build: check-go tidy
+	CGO_ENABLED=0 GOOS=$(OS) GOARCH=$(ARCH) go build -ldflags="-s -w -X main.version=$(VERSION)" -o $(PLUGIN_DIR)/$(BINARY) $(PKG)
+
+install: build
+	@echo "Built $(PLUGIN_DIR)/$(BINARY) (register it with: vault plugin register -sha256=<sha> secret $(BINARY))"
+
+test: check-go
+	go test -race -count=1 ./...
+
+lint: check-go
+	go vet ./...
+
+fmt: check-go
+	gofmt -w .
+
+tidy:
+	go mod tidy
+
+clean:
+	rm -rf $(PLUGIN_DIR)
+
+# Build the plugin binary then package it into an RPM with nfpm.
+rpm: build rpm-package
+
+# Package an already-built binary into an RPM (used by CI after the build step).
+rpm-package:
+	./scripts/build-rpm.sh $(VERSION)
+
+_LATEST := $(shell git tag --sort=-v:refname | grep -E '^v[0-9]+\.[0-9]+\.[0-9]+$$' | head -1)
+_BASE   := $(if $(_LATEST),$(_LATEST),v0.0.0)
+_MAJ    := $(shell echo $(_BASE) | sed 's/^v//' | cut -d. -f1)
+_MIN    := $(shell echo $(_BASE) | sed 's/^v//' | cut -d. -f2)
+_PAT    := $(shell echo $(_BASE) | sed 's/^v//' | cut -d. -f3)
+
+patch:
+	@NEW=v$(_MAJ).$(_MIN).$(shell expr $(_PAT) + 1); \
+	git tag $$NEW && echo "Tagged $$NEW" && git push origin $$NEW
+
+minor:
+	@NEW=v$(_MAJ).$(shell expr $(_MIN) + 1).0; \
+	git tag $$NEW && echo "Tagged $$NEW" && git push origin $$NEW
+
+major:
+	@NEW=v$(shell expr $(_MAJ) + 1).0.0; \
+	git tag $$NEW && echo "Tagged $$NEW" && git push origin $$NEW