feat: Add the vantagectl module and its cobra root

Its own module rather than a package under shared, so cobra and pflag stay
out of the module graphs of server, admin and sitesvc, which never use
them.

main.go

@@ -0,0 +1,19 @@
+// Command vantagectl backs up and restores a Vantage control plane.
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"gitea.hostxtra.co.uk/mrhid6/vantage/vantagectl/internal/cmd"
+)
+
+// Version is stamped at build time with -ldflags "-X main.Version=...".
+var Version = "dev"
+
+func main() {
+	if err := cmd.Execute(Version); err != nil {
+		fmt.Fprintln(os.Stderr, "error:", err)
+		os.Exit(1)
+	}
+}