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/CHANGELOG.md

@@ -1,5 +1,16 @@
 # Felhom Hub — Changelog
 
+## (no version bump) — slice 9 cross-repo wire-contract: `host.cpu_temp_c` (2026-06-10)
+
+Slice 9 adds a nullable **`cpu_temp_c`** field to the shared `HostMetrics` wire struct (the agent's
+new CPU/chassis-temperature collector). The agent's host-report carries it too, so the hub's
+**cross-repo host-report golden** (`internal/api/testdata/host-report.golden.json`) was updated to
+stay **byte-identical** with `felhom-agent/internal/hub/testdata/host-report.golden.json` (the
+duplicated-contract discipline; manual diff confirmed identical). **No hub code change** — the full
+report_json already persists the field verbatim, and the hub does not surface CPU temp on the
+operator dashboard yet (an optional later freebie). The golden-contract test (`host_test.go`) still
+passes (the host parse-struct ignores the extra key).
+
 ## v0.8.0 — opaque PBS recovery-code escrow storage (slice 7, doc 03 §8a) (2026-06-10)
 
 Hub half of slice-7 close-out: store the agent's **opaque** `R`-wrapped PBS-key escrow blob. The