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/testdata/host-report.golden.json

@@ -12,7 +12,8 @@
     "disk_used_bytes": 30000000000,
     "disk_percent": 19.7,
     "loadavg": ["0.10", "0.20", "0.15"],
-    "uptime_seconds": 86400
+    "uptime_seconds": 86400,
+    "cpu_temp_c": 47
   },
   "guests": [
     {