fix(agent): slice-3 follow-ups — keep run-status on config fail, selftest usage, contract golden (v0.3.1)

- collect: a per-guest GuestConfig failure preserves the ListLXC run-status (only
  spec dropped); empty status normalized to "unknown". Test asserts preserved
  "running" + nil spec.
- main: --selftest usage error now reads (want read|task|hub).
- contract: testdata/host-report.golden.json + TestHostReport_ContractMatchesGolden
  (field-name key-set check vs golden; byte-identical with the hub copy).
- version 0.3.0 -> 0.3.1.

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

internal/hub/testdata/host-report.golden.json

@@ -0,0 +1,38 @@
+{
+  "host_id": "demo-host-01",
+  "reported_at": "2026-06-08T12:00:00Z",
+  "agent_version": "0.3.1",
+  "host": {
+    "node": "demo-felhom",
+    "cpu_percent": 3.2,
+    "memory_total_bytes": 16777216000,
+    "memory_used_bytes": 4194304000,
+    "memory_percent": 25,
+    "disk_total_bytes": 152000000000,
+    "disk_used_bytes": 30000000000,
+    "disk_percent": 19.7,
+    "loadavg": ["0.10", "0.20", "0.15"],
+    "uptime_seconds": 86400
+  },
+  "guests": [
+    {
+      "vmid": 100,
+      "name": "felhom-cust-acme",
+      "status": "running",
+      "controller_version": "",
+      "spec": { "cores": 2, "memory_bytes": 2147483648, "disk_bytes": 21474836480 }
+    },
+    {
+      "vmid": 101,
+      "name": "felhom-cust-beta",
+      "status": "stopped",
+      "controller_version": ""
+    }
+  ],
+  "storage_targets": [],
+  "backups": [],
+  "restore_tests": [],
+  "pbs_snapshots": [],
+  "cloudflared": { "status": "active" },
+  "audit_tail": []
+}