make stderr capturing work for "streaming" job output

[JeanMertz]

I noticed when asking the LLM to run a command, my cargo build errors weren't being logged and instead the response was:

There was an error from the command cargo build:

Tool failed with code 101 and no output

With this change, the error is correctly captured and added to the prompt:

There was an error from the command cargo build:

bunch of stderr output here...

This might not be the correct way to solve the issue, but in the logs I always see the err variable as nil, and Plenary docs aren't very helpful on understanding what the variable represents. Feel free to close this and solve it the proper way, but this worked on my machine.

[olimorris]

Great stuff thanks for this.

At some point this week I'm going to move tools from leveraging Job.new():start() as I want to be strict over the order of completion