build: add Makefile with 'make build' gate

CLAUDE.md references 'make build' (lint + govulncheck + test) but no
Makefile existed. Add one whose build target runs tidy, vet, compile,
govulncheck, and test (a superset of CI plus govulncheck), with fmt,
lint (golangci-lint), and binary as separate targets.

Author: changkun

Makefile

@@ -0,0 +1,47 @@
+# Verification entry points for latere-cli.
+#
+# `make build` is the gate referenced in CLAUDE.md: lint + govulncheck + test.
+# It is a superset of CI (.github/workflows/ci.yaml runs tidy/vet/build/test),
+# adding govulncheck. Run it before committing.
+
+GO ?= go
+
+.PHONY: build
+build: tidy vet compile vuln test ## Full verification gate (run before committing)
+
+.PHONY: tidy
+tidy: ## Fail if go.mod/go.sum are not tidy
+	$(GO) mod tidy -diff
+
+.PHONY: vet
+vet: ## go vet
+	$(GO) vet ./...
+
+.PHONY: compile
+compile: ## Compile all packages
+	$(GO) build ./...
+
+.PHONY: test
+test: ## Run tests
+	$(GO) test ./...
+
+.PHONY: vuln
+vuln: ## Scan for known vulnerabilities
+	$(GO) run golang.org/x/vuln/cmd/govulncheck@latest ./...
+
+.PHONY: fmt
+fmt: ## Format all Go sources in place
+	gofmt -w .
+
+.PHONY: lint
+lint: ## Stricter lint via golangci-lint (has pre-existing findings; not in `make build`)
+	golangci-lint run ./...
+
+.PHONY: binary
+binary: ## Build the latere binary into ./latere
+	$(GO) build -o latere ./cmd/latere
+
+.PHONY: help
+help: ## List targets
+	@grep -hE '^[a-zA-Z_-]+:.*?## ' $(MAKEFILE_LIST) | \
+		awk 'BEGIN{FS=":.*?## "}{printf "  \033[36m%-12s\033[0m %s\n", $$1, $$2}'