slice 9: GET /host/metrics + CPU/chassis-temp collector (v0.14.0)

Add a host-wide, token-authed GET /host/metrics local-API endpoint that
re-serves the slice-4 collector's host + per-storage view to the customer
(the de-privileged controller can't read the host itself). Add the one new
collector — CPU/chassis temperature via sysfs hwmon/thermal-zones, graceful-
null — to the shared HostMetrics struct, so the hub report carries cpu_temp_c
too. Cross-repo host-report golden updated byte-identical.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/hub/report.go

@@ -38,6 +38,12 @@ type HostMetrics struct {
 	DiskPercent      float64  `json:"disk_percent"`
 	LoadAvg          []string `json:"loadavg"` // array of STRINGS (PVE shape)
 	UptimeSeconds    int64    `json:"uptime_seconds"`
+	// CPUTempC is the host CPU/chassis temperature in whole °C, or null when the hardware
+	// exposes no usable sensor (a headless VM, an unsupported board, or any read error all
+	// degrade to null — a missing sensor never fails the report). Same nullable contract as
+	// the per-disk SmartSummary.TemperatureC. Sourced from sysfs (hwmon / thermal zones).
+	// Cross-repo wire field (slice 9) — the hub's HostMetrics copy + golden carry it too.
+	CPUTempC *int `json:"cpu_temp_c"`
 }
 
 // Guest is one LXC. The agent reports vmid; the hub derives the guest PK