fix(cli): keep proxy listener alive in --proxy-only mode

[fl64]

Description

d8 v vnc --proxy-only printed the port and exited immediately, so the VNC listener was gone before any viewer could connect. The proxy-only goroutine closed doneChan right after printing the port, causing Run to return.

Now it waits for the context to be canceled, so the listener stays up until the user interrupts the command.

Why do we need it, and what problem does it solve?

--proxy-only was unusable: users could not connect an external VNC viewer to the printed port because the proxy was already dead.

What is the expected result?

1. d8 v vnc <vm> --proxy-only --port 5901 prints {"port":5901} and stays running.
2. A VNC viewer connecting to 127.0.0.1:5901 reaches the VM.
3. Ctrl-C stops the proxy cleanly.

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: cli
type: fix
summary: "d8 v vnc --proxy-only now keeps the VNC proxy listener alive so external viewers can connect to the printed port."
impact_level: low