ctxscope v1.0.1 — doc correction: plain slog calls keep the global tier

A plain slog.Info loses the per-context tier but keeps the global one, which
never came from a context. The docs said it carried no scope at all.

No behaviour change. Adds the two tests that would have caught it, including
the first coverage of the slog.SetDefault + package-level slog.InfoContext
setup the README recommends.