feat(host_agent): a root helper the agent can start, for updates, reboots and drains only (#5766)

#5766 "A root helper the agent can start, for a fixed set of maintenance
actions and nothing else", milestone 514.

The agent runs unprivileged under NoNewPrivileges, so it can't update or
reboot its host. It now leaves a request in /var/lib/steward-agent/requests,
the one path its unit lets it write; steward-maint.path starts maint.py as
root, which reads it without following links, accepts only update, reboot,
drain or undrain, deletes it, and writes the outcome to
/var/lib/steward-maint/results. The agent (1.17.0) reports each new result
state once as a `maintenance` event, re-checks pending updates when an update
finishes, and says in its metadata whether the helper is installed.

The units and the helper are served from one copy at
/plugins/host_agent/files/<name> (a fixed list), and install.sh plus the
install, provision and update playbooks all fetch them, replacing three
inline copies of the unit. Update agent installs the helper on existing
hosts; the Agent panel says under Manage agent whether it is there.

Steward handing out jobs is #5767; the cadence is #5768.

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

plugins/host_agent/templates/install.sh.j2

@@ -11,7 +11,9 @@ AGENT_VERSION="{{ agent_version }}"
 AGENT_USER="steward-agent"
 AGENT_DIR="/usr/local/lib/steward-agent"
 CONF_FILE="/etc/steward-agent.conf"
-UNIT_FILE="/etc/systemd/system/steward-agent.service"
+UNIT_DIR="/etc/systemd/system"
+UNITS="steward-agent.service steward-maint.service steward-maint.path"
+FILES_URL="${STEWARD_URL}/plugins/host_agent/files"
 
 # ── preflight ────────────────────────────────────────────────────────────────
 [ "$(id -u)" = "0" ] || { echo "must run as root (use sudo)"; exit 1; }
@@ -20,9 +22,10 @@ command -v python3   >/dev/null 2>&1 || { echo "python3 not found — install py
 
 # Handle --uninstall
 if [ "${1:-}" = "--uninstall" ]; then
-    systemctl disable --now steward-agent.service 2>/dev/null || true
-    rm -f "$UNIT_FILE" "$CONF_FILE"
-    rm -rf "$AGENT_DIR"
+    systemctl disable --now steward-maint.path steward-agent.service 2>/dev/null || true
+    for u in $UNITS; do rm -f "$UNIT_DIR/$u"; done
+    rm -f "$CONF_FILE"
+    rm -rf "$AGENT_DIR" /var/lib/steward-agent /var/lib/steward-maint
     systemctl daemon-reload
     # keep the system user — removing it risks orphaned files elsewhere
     echo "uninstalled"
@@ -36,7 +39,7 @@ fi
 
 # ── drop the agent file ──────────────────────────────────────────────────────
 mkdir -p "$AGENT_DIR"
-curl -sSL "${STEWARD_URL}/plugins/host_agent/agent.py" -o "$AGENT_DIR/agent.py"
+curl -fsSL "${STEWARD_URL}/plugins/host_agent/agent.py" -o "$AGENT_DIR/agent.py"
 chmod 0755 "$AGENT_DIR/agent.py"
 chown root:root "$AGENT_DIR/agent.py"
 
@@ -53,31 +56,24 @@ EOF
 chown "root:$AGENT_USER" "$CONF_FILE"
 chmod 0640 "$CONF_FILE"
 
-# ── write systemd unit ───────────────────────────────────────────────────────
-cat > "$UNIT_FILE" <<EOF
-[Unit]
-Description=Steward host agent
-After=network-online.target
-Wants=network-online.target
-
-[Service]
-Type=simple
-User=$AGENT_USER
-ExecStart=/usr/bin/python3 $AGENT_DIR/agent.py
-Restart=on-failure
-RestartSec=10
-NoNewPrivileges=yes
-ProtectSystem=strict
-ProtectHome=yes
-PrivateTmp=yes
-ReadOnlyPaths=/proc /sys
-
-[Install]
-WantedBy=multi-user.target
-EOF
+# ── the maintenance helper and the units (#5766) ─────────────────────────────
+# One copy, served by Steward, the same the playbooks install. The agent may
+# write only its request directory; the helper (root) and its results don't
+# belong to it.
+curl -fsSL "${FILES_URL}/maint.py" -o "$AGENT_DIR/maint.py"
+chmod 0755 "$AGENT_DIR/maint.py"
+chown root:root "$AGENT_DIR/maint.py"
+install -d -o root -g root -m 0755 /var/lib/steward-agent /var/lib/steward-maint /var/lib/steward-maint/results
+install -d -o "$AGENT_USER" -g "$AGENT_USER" -m 0700 /var/lib/steward-agent/requests
+for u in $UNITS; do
+    curl -fsSL "${FILES_URL}/$u" -o "$UNIT_DIR/$u"
+    chmod 0644 "$UNIT_DIR/$u"
+done
 
 systemctl daemon-reload
-systemctl enable --now steward-agent.service
+systemctl enable --now steward-maint.path
+systemctl enable steward-agent.service
+systemctl restart steward-agent.service
 
 echo
 echo "Steward host agent $AGENT_VERSION installed and running."