- Fixes#1509. The CLI's own help text tells users to
run `container help <subcommand>`, but every form
of that results in an error.
- Added a captured subcommand path, walked
Application`'s `subcommands` + `groupedSubcommands`
tree (matching `commandName` and `aliases`), and
printed `Application.helpMessage(for:)` for the resolved
target. Empty path keeps existing plugin-aware top-level
help; unknown path throws `ValidationError`.
AJ Emory