fix(channels): clear failure kind after recovery

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/channels/health.go

@@ -304,6 +304,7 @@ func mergeChannelHealth(previous, snapshot ChannelHealth) ChannelHealth {
 		}
 		snapshot.ConsecutiveFailures = 0
 		snapshot.FirstFailedAt = time.Time{}
+		snapshot.FailureKind = ""
 		if snapshot.LastFailedAt.IsZero() {
 			snapshot.LastFailedAt = previous.LastFailedAt
 		}

internal/channels/health_test.go

@@ -504,6 +504,9 @@ func TestMergeChannelHealthTracksFailureTimelineAndRecovery(t *testing.T) {
 	recovered.CheckedAt = recoveredAt
 	recovered = mergeChannelHealth(second, recovered)
 
+	if recovered.FailureKind != "" {
+		t.Fatalf("expected failure kind to clear after recovery, got %q", recovered.FailureKind)
+	}
 	if recovered.ConsecutiveFailures != 0 {
 		t.Fatalf("expected consecutive failures reset, got %d", recovered.ConsecutiveFailures)
 	}