docs(agent): note the NVIDIA Container Toolkit host prereq

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

agent/README.md

@@ -7,6 +7,17 @@ Redis stay private; the agent never touches them.
 
 You run it when you want a burst and stop it to reclaim the card.
 
+## 0. Host prerequisite — NVIDIA Container Toolkit
+Docker needs the toolkit to hand the GPU to a container (else: *"could not select
+device driver nvidia with capabilities [[gpu]]"*). On Arch/CachyOS:
+```sh
+sudo pacman -S nvidia-container-toolkit
+sudo nvidia-ctk runtime configure --runtime=docker
+sudo systemctl restart docker
+# verify:
+docker run --rm --gpus all nvidia/cuda:12.4.1-base-ubuntu22.04 nvidia-smi
+```
+
 ## 1. Get a token
 In FC: **Settings → Tagging → GPU agent → Generate token** (or Rotate). Copy it.