feat(railiance): implement CUST-WP-0032 Haskell build machine infra

Packer build definition, cloud-init autoinstall, GHCup toolchain script,
boot-time registration agent (state-hub + autossh dual tunnel), systemd
unit, key injection, remote-build Makefile, smoke test, and deployment
README. All 15 tasks complete.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

infra/build-machines/haskell/scripts/install-agent.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+set -euo pipefail
+
+# Copy agent artefacts (provisioned by Packer file provisioner)
+install -m 0755 /tmp/build-agent.py /usr/local/bin/build-agent
+install -m 0644 /tmp/build-agent.service /etc/systemd/system/build-agent.service
+install -m 0600 /tmp/build-agent.env.template /etc/build-agent.env.template
+
+# Placeholder env file — operator fills this in before first boot
+if [ ! -f /etc/build-agent.env ]; then
+  cp /etc/build-agent.env.template /etc/build-agent.env
+fi
+
+# Install autossh
+apt-get install -y -qq autossh
+
+# Enable agent service (starts on boot, after network-online)
+systemctl daemon-reload
+systemctl enable build-agent.service
+
+# SSH host key generation (deterministic at first boot, not baked in image)
+dpkg-reconfigure openssh-server