Help argument order and parsing

Even though we're expecting `buildkite-agent command help` we should probably handle this better:

```
$ buildkite-agent help artifact

Options:

```

Start by reproducing `buildkite-agent help artifact` and trace the command-line argument parsing and help entry point in the Go agent. The issue does not name files or tests, so first determine how `buildkite-agent command help` and the reversed argument order are handled. Done means the help invocation no longer produces the empty or malformed output shown.

Written by the indexing model from the issue text.