UI improvements for one-shot mode (--disconnect-after-job)

I'm using `buildkite-agent` with `--disconnect-after-job` for containerization reasons, running all of buildkite-agent under sandbox-exec or docker. In this scenario, the WebUI always reports at most one job per agent, as apparently reconnecting the agent (which I do after every job) results in a new agent ID.

For example, every idle agent reports:

... while an active agent will only report a single job, i.e., the one it's currently running:

This is not great, as it complicates debugging worker issues, with no easy way to inspect all runs by this agent.
I'd be happy to have to specify my own UID for this, so that the WebUI can correctly group runs.