(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 a bash double fork.

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 :-)

[zeylos]

• I have moved from nohup/disown to a double fork which is way cleaner and totally unlinked to the current shell (this caused trouble when used with other background jobs running in the shell).
• I double checked and the port-forward is indeed done automatically on the same port even on linux - although the --port-forward flag could still be usefull on agent-vm to forward to another port (e.g. --port-forward 80:3000) it may not really be needed for agent-vm, I can remove this flag.
• I tested to only launch opencode serve and background the entire agent-vm command (agent-vm opencode serve --port 3000 &) but it sometimes break the shell, the process is entirely linked to the current shell (which is weird for the opencode serve setup which is then probably used with the webapp or desktop app) and the overall experience isn't really clean. I still think the --background flag provides a way better user experience than just backgrounding the agent-vm command and would be useful to have in agent-vm