feat(vm): add --gui/--nogui to vm new --start, default headless

Mirror the `job` flag on the manual lifecycle: `vm new --start` was
hardcoded headless; add --gui/--nogui (default --nogui) so a clone can be
booted into the Workstation GUI window for interactive debugging. The
start banner reflects the mode.

Also update the runbook: §4.1 shows the --gui variant, and §3 notes that
the burnin-dummy build.sh is a ~30s fake build writing dist/build-info.txt
so the examples exercise the full pipeline.

src/ci_orchestrator/commands/vm.py

@@ -406,7 +406,15 @@ def vm_cleanup(
     "start_vm",
     is_flag=True,
     default=False,
-    help="Start the clone headless immediately after creation.",
+    help="Start the clone immediately after creation.",
+)
+@click.option(
+    "--gui/--nogui",
+    "gui",
+    default=False,
+    show_default=True,
+    help="With --start, open the Workstation GUI window (--gui, needs an X "
+    "display) or start headless (--nogui, the default).",
 )
 def vm_new(
     template: str,
@@ -416,6 +424,7 @@ def vm_new(
     vmrun_path: str | None,
     guest_os: str,
     start_vm: bool,
+    gui: bool,
 ) -> None:
     """Create a linked clone of the template VM for an ephemeral CI build.
 
@@ -475,9 +484,10 @@ def vm_new(
     click.echo(f"[vm new] clone created in {elapsed:.1f}s", err=True)
 
     if start_vm:
-        click.echo("[vm new] starting VM headless...", err=True)
+        mode = "GUI" if gui else "headless"
+        click.echo(f"[vm new] starting VM ({mode})...", err=True)
         try:
-            backend.start(handle, headless=True)
+            backend.start(handle, headless=not gui)
         except BackendError as exc:
             raise click.ClickException(f"vmrun start failed: {exc}") from exc
         click.echo("[vm new] VM started.", err=True)