rules.go: add context.TODO

[cristaloleg]

// TODO returns a non-nil, empty Context. Code should use context.TODO when
// it's unclear which Context to use or it is not yet available (because the
// surrounding function has not yet been extended to accept a Context
// parameter). TODO is recognized by static analysis tools that determine
// whether Contexts are propagated correctly in a program.
func TODO() Context {
	return todo
}

But last sentence was removed from the documentation in https://go-review.googlesource.com/c/go/+/130876/

Also issues golang/lint#227 & golang/go#16742