Add the Go module, the gate and the config package

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

Makefile

@@ -0,0 +1,17 @@
+# crossbar gate. `make gate` must pass before any task is called done. It needs no network.
+
+.PHONY: gate build smoke
+
+gate:
+	@test -z "$$(gofmt -l . 2>&1)" || { echo "gofmt: these files need formatting:"; gofmt -l .; exit 1; }
+	go vet ./...
+	go test -race -count=1 ./...
+	sh scripts/check-lines.sh
+	@echo "gate: ok"
+
+build:
+	go build -o bin/ ./cmd/...
+
+# Runs the whole thing against two fake upstreams. Task 05 brings the script.
+smoke: build
+	sh tools/smoke.sh