cli: resolve subcommand path in container help

[0xMH]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Fixes #1509. The CLI's own help text tells users to run container help <subcommand>, but every form of that errored:

$ container help image delete
Error: 2 unexpected arguments: 'image', 'delete'
$ container help run
Error: Unexpected argument 'run'

The project's custom HelpCommand (registered so plugins show up in top-level help) had no positional @Argument, so swift-argument-parser routed help <x> to it and rejected the trailing tokens. 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.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

[github-actions]

Code Coverage

Tier	Line Coverage
Unit	33.49%
Integration	19.62%
Combined	52.56%

[jglogan]

Looks really good @0xMH! Thanks for the contribution.

[0xMH]

You are welcome @jglogan. Thanks for reviewing!