chore: add Makefile with check gate, golangci-lint config, dockerignore

- make check runs lint + vet + test-short; lint is pinned to the go.mod
  toolchain so it works when the host Go is newer than golangci-lint's
- gofumpt main.go and patch.go; drop unused gatewayGroup/gatewayKind vars

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Makefile

@@ -0,0 +1,56 @@
+BINARY  := manager
+BIN_DIR := bin
+IMG     ?= gateway-cert-operator:dev
+
+# GIT_COMMIT is baked into the image (-ldflags in the Dockerfile); -dirty
+# covers staged and untracked changes too, which `git diff --quiet` misses.
+GIT_COMMIT ?= $(shell git rev-parse --short=12 HEAD 2>/dev/null || echo unknown)$(shell test -z "$$(git status --porcelain 2>/dev/null)" || echo -dirty)
+
+# golangci-lint type-checks the standard library of whatever toolchain is active,
+# and fails on one newer than the Go it was built with. Linting with the go.mod
+# version keeps it working when the host Go is ahead, and matches CI.
+GO_MOD_VERSION := $(shell awk '$$1 == "go" {print $$2}' go.mod)
+
+.PHONY: all build run test test-short lint vet fmt tidy check docker-build clean help
+
+all: check build
+
+build: ## Build the manager binary to bin/manager
+	mkdir -p $(BIN_DIR)
+	go build -o $(BIN_DIR)/$(BINARY) .
+
+run: ## Run the manager against the current kubeconfig context
+	go run .
+
+test: ## Run the full test suite with the race detector
+	go test -race -cover ./...
+
+test-short: ## Run unit tests only — skips tests guarded by testing.Short()
+	go test -short -cover ./...
+
+lint: ## Run golangci-lint
+	GOTOOLCHAIN=go$(GO_MOD_VERSION) golangci-lint run ./...
+
+vet: ## Run go vet
+	go vet ./...
+
+fmt: ## Format all Go files with gofumpt
+	gofumpt -w .
+
+tidy: ## Tidy go.mod and go.sum
+	go mod tidy
+
+check: lint vet test-short ## Run all quality checks (lint + vet + test-short)
+
+docker-build: ## Build the container image (IMG=name:tag)
+	docker build --build-arg GIT_COMMIT=$(GIT_COMMIT) -t $(IMG) .
+
+clean: ## Remove build artifacts
+	rm -rf $(BIN_DIR)
+
+help: ## Show available targets
+	@echo 'Usage: make <target>'
+	@echo ''
+	@echo 'Targets:'
+	@grep -E '^[a-zA-Z_-]+:.*##' $(MAKEFILE_LIST) | \
+		awk 'BEGIN {FS = ":.*## "}; {printf "  \033[36m%-14s\033[0m %s\n", $$1, $$2}'