feat: add --detach flag to browserkit start

[jonzarecki]

Summary

browserkit start currently runs in the foreground, requiring the user to manage backgrounding manually (e.g. nohup ... &).

Add a --detach (or -d) flag so the daemon forks into the background automatically:

browserkit start --detach --config browserkit.config.js

Expected behavior

• Forks the process into the background
• Writes logs to a default location (e.g. ~/.browserkit/daemon.log) or a configurable --log-file path
• Prints the PID and log path on exit
• browserkit stop / browserkit status already exist and should work with the detached process

Motivation

When starting the daemon from an IDE terminal or a script, the foreground process holds the terminal. Users currently have to wrap it in nohup ... & which is easy to forget and doesn't integrate with the existing pidfile/status infrastructure.