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.go

@@ -105,13 +105,18 @@ func (c *Collector) collectGuests(ctx context.Context) []Guest {
 	guests := make([]Guest, 0, len(lxc))
 	for _, g := range lxc {
 		entry := Guest{VMID: g.VMID, Name: g.Name, Status: g.Status, ControllerVersion: ""}
+		// Normalize an empty run-status to "unknown" so the wire value is always one
+		// of running|stopped|unknown (matches the hub handler's empty→unknown default).
+		if entry.Status == "" {
+			entry.Status = "unknown"
+		}
 		// GuestConfig supplies cores; memory/disk come from the list entry (bytes).
+		// On failure, KEEP the known run-status from ListLXC — only the spec is lost.
 		cfg, err := c.px.GuestConfig(ctx, g.VMID)
 		if err != nil {
-			c.logger.Warn("hub: GuestConfig failed; guest degraded to unknown",
+			c.logger.Warn("hub: GuestConfig failed; spec omitted (run-status kept)",
 				"vmid", g.VMID, "err", err)
-			entry.Status = "unknown"
-			entry.Spec = nil // omitted
+			entry.Spec = nil
 		} else {
 			entry.Spec = &GuestSpec{
 				Cores:       cfg.Cores,