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/__init__.py

@@ -86,10 +86,22 @@ def get_alert_metrics() -> dict[str, list[str]]:
     the whole host ("<host>") or one interface ("<host>:net:<iface>") or disk
     ("<host>:diskio:<device>").
 
+    Drive health (agent ≥ 1.20.0, #5789), read hourly with smartctl by the
+    root helper and judged in drives.py, per drive on "<host>:drive:<device>":
+    drive_failing is 1 when the drive fails its own self-assessment, raises
+    an NVMe critical warning or runs out of spare blocks; drive_life_used_pct
+    is how much of its rated life it has used; drive_bad_sectors counts its
+    media errors or reallocated / pending / uncorrectable sectors;
+    drive_temp_c is its own temperature. Per host, for covering rules:
+    drives_failing (how many) and drive_life_used_max_pct (the most worn):
+    "drive failing" is drives_failing > 0, "drive wearing out" is
+    drive_life_used_max_pct > 90.
+
     Recorded but not offered (NOT_ALERTABLE): disk_used_bytes and
     disk_total_bytes, where disk_used_pct is the figure to watch (and docker
     uses disk_total_bytes for something else), and mem_cached_bytes /
     mem_buffers_bytes, parts of what mem_available_bytes already counts."""
+    from .drives import DRIVE_METRICS
     return {"host_agent": [
         "cpu_pct", "mem_used_pct", "mem_available_bytes", "swap_used_bytes",
         "disk_used_pct_worst", "disk_used_pct", "inodes_used_pct", "load_1m", "load_5m", "load_15m",
@@ -98,6 +110,7 @@ def get_alert_metrics() -> dict[str, list[str]]:
         "agent_rss_bytes", "agent_cpu_pct",
         "updates_pending", "updates_security", "reboot_required", "maintenance_failed",
         "temp_c_max", "temp_c", "net_rx_bps", "net_tx_bps", "disk_read_bps", "disk_write_bps",
+        *DRIVE_METRICS,
     ]}