Allow to pass test command to the agent

[ztxtz]

Just like with /run, the /agent should accept a command that it will run to test changes.

Currently, it's not very useful as it always auto-selects the wrong way to test it.

[jakethekoenig]

You can tell the model which command to run in your prompt and it will generally respect it. I can see the value of having a place to explicitly put it to make it easier on the model though.

[PCSwingle]

You can already do this! Just say no to the model suggested commands, and then enter your own commands.