feat(tui-go): health-checks pane

Surface the health subsystem (llama/event-store/disk probes) in the TUI,
mirroring the session-stats pane: ClientMessage.GetHealthChecks ->
ServerMessage.HealthChecks (health.checks, reuses HealthReport) ->
StreamQueries.healthChecks via HealthInspectionService; Go OverlayHealth on
key H + palette 'health checks', pull-based fetch, per-subject status with
degraded loud. Empty/disabled -> visible fallback (no blank/lie). Go+Kotlin
golden tests pin the wire contract field-for-field. Observability spec §4/§3.

apps/tui-go/internal/app/demo.go

@@ -152,6 +152,14 @@ func PreviewFrame(kind string, w, h int) string {
 		m.overlay = OverlayStats
 		m.statsFor = "04a546aa"
 		m.stats = sampleStats()
+
+	case "health":
+		m.connected = true
+		m.currentModel = "llama-cpp:default"
+		m.sessions = sampleSessions()
+		m.selectedID = "04a546aa"
+		m.overlay = OverlayHealth
+		m.health = sampleHealth()
 	}
 	return m.View()
 }
@@ -193,6 +201,39 @@ func sampleStats() *protocol.StatsDto {
 	}
 }
 
+func sampleHealth() *protocol.HealthDto {
+	return &protocol.HealthDto{
+		Overall: "DEGRADED",
+		Subjects: []protocol.HealthSubjectDto{
+			{
+				Subject:       "DISK",
+				Status:        "DEGRADED",
+				Metric:        "free_bytes",
+				ObservedValue: 512_000_000,
+				Detail:        "disk free below 1 GB threshold",
+				Since:         "2026-06-14T10:00:00Z",
+			},
+			{
+				Subject:       "EVENT_STORE",
+				Status:        "HEALTHY",
+				Metric:        "write_latency_ms",
+				ObservedValue: 4,
+				Detail:        "ok",
+				Since:         "2026-06-14T09:00:00Z",
+			},
+			{
+				Subject:       "LLAMA_SERVER",
+				Status:        "HEALTHY",
+				Metric:        "http_status",
+				ObservedValue: 200,
+				Detail:        "ok",
+				Since:         "2026-06-14T08:30:00Z",
+			},
+		},
+		CheckedAt: "2026-06-14T10:05:00Z",
+	}
+}
+
 func sampleWorkflows() []Workflow {
 	return []Workflow{
 		{ID: "healthcheck", Description: "ping endpoints and report status"},