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/collect_test.go

@@ -58,7 +58,7 @@ func TestCollect_HostAndGuests(t *testing.T) {
 	}
 }
 
-func TestCollect_GuestConfigFailureDegradesButStillReports(t *testing.T) {
+func TestCollect_GuestConfigFailureKeepsStatusOmitsSpec(t *testing.T) {
 	px := &fakePx{
 		node: "demo-felhom",
 		ns:   newTestNodeStatus(),
@@ -69,7 +69,7 @@ func TestCollect_GuestConfigFailureDegradesButStillReports(t *testing.T) {
 		cfg:    map[int]proxmox.GuestConfig{100: {Cores: 2}},
 		cfgErr: map[int]error{200: errors.New("config read failed")},
 	}
-	c := NewCollector(px, fakeProber{status: "active"}, "h", "0.3.0", quietLogger())
+	c := NewCollector(px, fakeProber{status: "active"}, "h", "0.3.1", quietLogger())
 	r, err := c.Collect(context.Background())
 	if err != nil {
 		t.Fatalf("a per-guest failure must NOT fail the whole report: %v", err)
@@ -77,9 +77,14 @@ func TestCollect_GuestConfigFailureDegradesButStillReports(t *testing.T) {
 	if len(r.Guests) != 2 {
 		t.Fatalf("guests = %d", len(r.Guests))
 	}
+	// GuestConfig failed for vmid 200, but its run-status (from ListLXC) is known and
+	// must be PRESERVED — only the spec is dropped.
 	bad := r.Guests[1]
-	if bad.Status != "unknown" || bad.Spec != nil {
-		t.Errorf("degraded guest = %+v (want status=unknown, spec=nil)", bad)
+	if bad.Status != "running" {
+		t.Errorf("status = %q, want preserved \"running\" (not forced to unknown)", bad.Status)
+	}
+	if bad.Spec != nil {
+		t.Errorf("spec = %+v, want nil (omitted on config failure)", bad.Spec)
 	}
 }