brintOS agent rig
=================

linux-6.12 glibc + bash + coreutils, plus the guest half of a headless agent
control channel.

Boot it with nothing installed but Node:

    node /boot/hardwarejs.js --fs <image root> --init /bin/bash

To give a host-side agent a control channel, boot with the agent runner and a
socket, then start the guest agent:

    node run-machine-agent.mjs --fs . --init /bin/bash --control-socket /tmp/a.sock
    # then, in the guest:
    /usr/local/bin/brintos-agent &

The host can now send "<mark> <command>" on the socket and read back the
output followed by the mark.

Notes learned the hard way
--------------------------
* virtio-console ports are SINGLE-OPEN — use one `exec 4<>` fd for both
  directions; opening twice fails silently.
* Deploying into a brintos-fs mount needs `rsync --inplace`. The default
  temp-file-then-rename pattern fails and rsync still exits 0 on some paths,
  so the deploy looks like it worked and nothing lands.
* Serial: `stty ... min 0 time 30` before reading /dev/ttySER0, or a blocking
  read wedges the machine and `timeout` will not reliably kill it.