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>

steward/core/formatting.py

@@ -352,6 +352,12 @@ METRICS: dict[str, tuple[str, str]] = {
     "net_tx_bps": ("Network out", "bytes per second sent, by the whole host or one interface"),
     "disk_read_bps": ("Disk reads", "bytes per second read, from all disks or one"),
     "disk_write_bps": ("Disk writes", "bytes per second written, to all disks or one"),
+    "drives_failing": ("Drives failing", "drives on the host failing their own health check, raising a critical warning or out of spare blocks"),
+    "drive_failing": ("Drive failing", "1 when this drive fails its own health check, raises a critical warning or is out of spare blocks"),
+    "drive_life_used_max_pct": ("Most worn drive", "the most life any of the host's SSDs has used of its rating"),
+    "drive_life_used_pct": ("Drive life used", "how much of its rated write life this drive has used"),
+    "drive_bad_sectors": ("Bad sectors", "media errors, or reallocated, pending and uncorrectable sectors, as the drive counts them"),
+    "drive_temp_c": ("Drive temperature", "the drive's own temperature sensor"),
 }
 
 # Where a rule's metric comes from, as the picker and the rules list say it.