Understanding BEP is not easy at first. Build whatever target you want to enhance with the flag --build_event_json_file=bep.json and inspect what's in there to get a better grasp at what events the code should react.
A mocked version of buildkite-agent cli is provided under //cmd/mockagent. It does nothing else that dumping its args and stdin in /tmp/_log_mock_agent.txt. Set the property buildkite_agent_path to its compiled path to tell the plugin to use that binary instead of buildkite-agent.
At some point, it's mandatory to test things against a real Buildkite build ran by an agent, which requires the plugin to be available. The repository is configured to build a release once a tag is pushed (vX.Y.Z-pre) so just push a tag and turn the automatically created draft release into a pre-release, which you can then use in any pipeline to test the result.
Demo
TODO: Consider showing off your new plugin with a little animated demo of your terminal! We highly recommend asciinema.