Remove the docker module: dotfiles no longer installs Docker

Installing Docker Engine is a host/image decision, not a per-user
environment one. In Coder workspaces (unprivileged pods, no systemd) the
module ran get.docker.com on every start for a daemon that can never
run there. Where Docker is wanted, install it on the host or bake it
into the image.

Also drops the zshrc newgrp-docker hook that existed only for this
module's group change, and the docs/skills references to docker/.
docker-rootless-cgroup/ (configures Kasm's existing rootless daemon)
and bin/'s kind gate (detects a daemon) stay.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.agents/skills/verify-module/SKILL.md

@@ -63,8 +63,8 @@ faithful rehearsal.
 
 Kasm has no systemd and no rootful Docker. Anything that would `systemctl` must
 no-op rather than abort — you can't fully rehearse this in a container, so read
-the code path and confirm the guard exists (`docker/install.sh` and
-`docker-rootless-cgroup/install.sh` are the references).
+the code path and confirm the guard exists (`docker-rootless-cgroup/install.sh`
+is the reference).
 
 ## Full run