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/files/steward-maint.service

@@ -1,5 +1,6 @@
 # Runs as root, and does only what maint.py's fixed list allows: update the
-# host's packages, reboot it, or drain / reactivate its swarm node (#5766).
+# host's packages, reboot it, drain / reactivate its swarm node (#5766), or
+# read its drives' health with smartctl (#5789).
 [Unit]
 Description=Steward maintenance helper