Add `--detached` to run Dev Proxy in the background

[waldekmastykarz]

Add the --detached option to run Dev Proxy in the background. This could help with agentic workflows to prevent agents from getting stuck on running Dev Proxy which is a long-running process. We'd need to consider how to expose logs. Checking status and closing Dev Proxy is already handled by the API.

[garrytrinder]

I like the idea 👍

If we were to follow existing approaches for handling detached processes, as an example, like like Docker CLI, we might want to consider adding supporting commands to make it easier to get information about the running background process. We have the API, but I think having lightweight wrapper commands that call the API would be useful.

Suggestions:

devproxy --detached          # Start in background, print API URL + PID
devproxy status              # Is it running? What port?
devproxy stop                # Stop it
devproxy logs                # Show logs (requires log file approach)

We should also consider updating our GitHub Actions to use this approach when implemented.

[waldekmastykarz]

Great idea to align with what's already out there! status and stop could basically be wrappers around the API. logs is the one that needs thinking

[waldekmastykarz]

Code is ready but waiting on #1508 to be merged so that we can align logging between interactive and detached mode