Add vintner doctor for diagnosing a broken wine/toolchain setup

A misconfigured environment (wine missing, msitools not installed, a
partially-built toolchain) otherwise only surfaces as a wine-specific
error buried deep inside a build. `vintner doctor` checks wine itself
(found and actually runs), the optional extraction tools download
needs, and every installed <dest>/bin/<arch> toolchain's on-disk
layout, printing a pass/fail checklist and exiting non-zero if
anything's broken.

cmd/vintner/main.go

@@ -50,6 +50,8 @@ func runCLI(args []string) int {
 		return runEnv(args[1:])
 	case "completion":
 		return runCompletion(args[1:])
+	case "doctor":
+		return runDoctor(args[1:])
 	case "version", "v", "--version":
 		fmt.Println(versionString())
 		return 0