feat: auto-deploy tagged releases to CT 126 via signed webhook (0.3.1)

CI's new deploy job (tags only) sends the house HMAC webhook to CT 126; the
listener runs /usr/lib/devdesk/devdesk-deploy, which waits for the version in
the apt registry and upgrades devdesk with --force-confold. Running sessions
are untouched. Hook setup is documented in docs/DEPLOY.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libexec/devdesk-deploy

@@ -0,0 +1,54 @@
+#!/bin/bash
+# Upgrade DevDesk from the LotusGuild apt registry. Run by the host's webhook
+# listener (hook "devdesk-deploy") when CI publishes a v* tag; the tag's ref is
+# passed as $1. Safe for running sessions: package upgrades restart only
+# devdesk.service (ttyd), never devdesk-tmux.service where Claude lives.
+set -euo pipefail
+
+LOG=${DEVDESK_DEPLOY_LOG:-/var/log/devdesk-deploy.log}
+SOURCES=${DEVDESK_APT_SOURCES:-sources.list.d/lotusguild.list}
+
+log() {
+    printf '%s %s\n' "$(date '+%F %T')" "$*" >>"$LOG"
+}
+
+exec 9>/run/devdesk-deploy.lock
+flock -w 900 9 || {
+    log "another deploy is still running; giving up"
+    exit 1
+}
+
+want=${1:-}
+want=${want#refs/tags/}
+want=${want#v}
+before=$(dpkg-query -W -f='${Version}' devdesk 2>/dev/null || echo none)
+log "=== deploy: installed $before, release ${want:-latest}"
+
+refresh() {
+    apt-get update -q -o Dir::Etc::sourcelist="$SOURCES" -o Dir::Etc::sourceparts=- \
+        -o APT::Get::List-Cleanup=0 >>"$LOG" 2>&1
+}
+
+# The registry can lag the release by a few seconds: wait for the tagged version.
+candidate=""
+for _ in $(seq 1 18); do
+    refresh || true
+    candidate=$(apt-cache policy devdesk | awk '/Candidate:/ {print $2}')
+    if [ -z "$want" ] || [ "$candidate" = "$want" ]; then
+        break
+    fi
+    sleep 10
+done
+
+if [ -n "$want" ] && [ "$candidate" != "$want" ]; then
+    log "registry still offers $candidate, not $want; giving up"
+    exit 1
+fi
+if [ "$candidate" = "$before" ]; then
+    log "already at $before; nothing to do"
+    exit 0
+fi
+
+DEBIAN_FRONTEND=noninteractive apt-get install -y -q --only-upgrade \
+    -o Dpkg::Options::=--force-confold devdesk </dev/null >>"$LOG" 2>&1
+log "devdesk $before -> $(dpkg-query -W -f='${Version}' devdesk)"