feat: add vm-headless app to run the VM detached in the background

[gotha]

Summary

Adds a vm-headless flake app so the VM can be started detached in the background, printing the SSH port to connect to instead of attaching a console to your terminal.

• nix run .#vm — unchanged (foreground; serial console attached to your terminal).
• nix run .#vm-headless — new; daemonizes QEMU and prints the SSH command, then returns control to your shell.
• nix run .#vm-gui — unchanged (graphical window).

How it works

• lib/mk-vm-runner.nix gains a background runner that launches QEMU with -daemonize -display none -monitor none, captures the serial console to /tmp/<vm>-console.log, and writes a pidfile to /tmp/<vm>.pid for easy stopping.
• lib/mk-vm-apps.nix exposes it as the vm-headless app.
• The SSH port is chosen the same way as the existing runners (random 20000–30000, also saved to /tmp/<vm>-ssh-port).

Example output:

VM 'dev-vm' started in background.

  SSH:         ssh dev@localhost -p 24817
  Port file:   /tmp/dev-vm-ssh-port
  Console log: /tmp/dev-vm-console.log
  Stop:        kill $(cat /tmp/dev-vm.pid)

Notes

• Independent of feat: pre-install project devShell packages at build time #5 (devShell prebuild); branched off origin/main so the diff is vm-headless only.

Test plan

• nix eval .#apps.x86_64-linux lists default, vm, vm-gui, vm-headless.
• Rendered runner shell passes bash -n.
• nix run .#vm-headless boots a VM detached and ssh dev@localhost -p <port> connects.
• nix run .#vm still attaches to the terminal as before.

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