Let a build agent be paused, have a build killed, and end an ask cancelled as it took it (hq ADR 0219)

A queued ask could only be waited out and a running build only ended by
stopping the machine, which redelivered it elsewhere. The holder now serves
current, kill, pause and resume on its own machine's subjects; a kill ends
the build's process group and labelled containers and settles the ask as
failed, killed by hand; pause is kept in the workspace across a restart and
said on the bus. The controller writes cancelled ids to a cancelled set the
holder reads on taking an ask, closing the race a delete alone leaves.

internal/builder/builder.go

@@ -761,6 +761,9 @@ func Command(ctx context.Context, dir, name string, args ...string) (string, err
 	tell("run", "$ (%s) %s %s", short(filepath.Base(dir)), name, strings.Join(args, " "))
 	cmd := exec.CommandContext(ctx, name, args...)
 	cmd.Dir = dir
+	// **Ended whole when the build is** (novox/hq ADR 0219): its own process group, killed as one
+	// when the context ends, so a build killed by hand leaves no `git` or `docker` child running.
+	inItsOwnGroup(cmd)
 	out, err := cmd.CombinedOutput()
 	if err != nil {
 		tell("run", "! %s %s failed after %s", name, args[0], since(started))