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>

cmd/felhom-agent/main.go

@@ -26,7 +26,7 @@ import (
 
 // version is the agent version. Overridable at build time with
 // -ldflags "-X main.version=<v>"; defaults to the in-repo CHANGELOG version.
-var version = "0.3.0"
+var version = "0.3.1"
 
 func main() {
 	var (
@@ -326,7 +326,7 @@ func (f *selftestFlag) Set(v string) error {
 	case "hub":
 		f.mode = "hub"
 	default:
-		return fmt.Errorf("invalid --selftest value %q (want read|task)", v)
+		return fmt.Errorf("invalid --selftest value %q (want read|task|hub)", v)
 	}
 	return nil
 }