slice 9 docs + wire-contract: host.cpu_temp_c golden + doc 03 GET /host/metrics

Update the cross-repo host-report golden byte-identical with felhom-agent
(host.cpu_temp_c). Document GET /host/metrics in doc 03 section 6 and define
slice 9 in the section 9 roadmap. No hub code change / no version bump.

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

hub/internal/api/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": [
     {