feat(host_agent): each drive's health, read with smartctl, alertable (#5789)

The root helper gains one read-only action, `drives`: it finds the drives
itself (smartctl --scan-open), reads each as JSON, and writes a summary the
agent carries on every sample (agent 1.20.0). Steward judges the figures in
drives.py (failing / worn / watch / ok / no SMART / unreadable), records them
per drive and as host roll-ups for covering rules, keeps the newest report on
the registration and lists it on the host page.

install-deps.sh installs smartmontools with apt, dnf, yum, zypper, pacman or
apk, run by install.sh and every agent playbook; one it can't install is
reported, never fatal.

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

plugins/host_agent/templates/install.sh.j2

@@ -63,6 +63,12 @@ chmod 0640 "$CONF_FILE"
 curl -fsSL "${FILES_URL}/maint.py" -o "$AGENT_DIR/maint.py"
 chmod 0755 "$AGENT_DIR/maint.py"
 chown root:root "$AGENT_DIR/maint.py"
+# Its packages (smartctl for drive health, #5789), with whichever package
+# manager this host has; one that can't be installed is said, never fatal.
+curl -fsSL "${FILES_URL}/install-deps.sh" -o "$AGENT_DIR/install-deps.sh"
+chmod 0755 "$AGENT_DIR/install-deps.sh"
+chown root:root "$AGENT_DIR/install-deps.sh"
+"$AGENT_DIR/install-deps.sh" || true
 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