Minor cli cleanup: show help when no command provided, hide login/deploy unless plugins installed

[pavelgj]

$ genkit help
Usage: genkit [options] [command]

Firebase Genkit CLI

Options:
  -V, --version                                       output the version number
  -h, --help                                          display help for command

Commands:
  start [options]
  flow:run [options] <flowName> [data]
  flow:batchRun [options] <flowName> <inputFileName>
  flow:resume <flowName> <flowId> <data>
  eval:extractData [options] <flowName>
  eval:run [options] <dataset>
  eval:flow [options] <flowName> [data]
  init [options]                                      Initialize a project directory with Genkit
  config
  help