(feat) added --port-forward and --background flags

[zeylos]

Hi,

What

This PR adds

• the --port-forward flag (mimicking the limactl --port-forward flag) to forward additional ports to the host
• the --background flag, which allows to run opencode/claude/codex or arbitrary commands with nohup and disown.

Why

This allows the user to do stuff like that to access the session from a Web browser, a Desktop app or an IDE on the host

agent-vm --port-forward '3000:3000' --background opencode serve --port 3000

Screenshots

?

• We may need to think if we want --background and --rm to be exclusive or not, for now it's explicitely exclusive because I don't see any usecase where we'd want both + it's probably tricky to get the exit event from the disowned process to trigger the destroy.
• Probably need some testing, I used it with success for 2 hours :-)