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/overlays.go

@@ -61,6 +61,8 @@ func (m Model) renderOverlay(base string) string {
 		return m.center(m.statsModal())
 	case OverlayIdeas:
 		return m.center(m.ideasModal())
+	case OverlayHealth:
+		return m.center(m.healthModal())
 	}
 	return base
 }
@@ -750,6 +752,66 @@ func (m Model) statsModal() string {
 	return t.Overlay.Width(w).Render(b.String())
 }
 
+func (m Model) healthModal() string {
+	t := m.theme
+	w := m.modalWidth()
+
+	var b strings.Builder
+	b.WriteString(m.titleLine("health checks"))
+	b.WriteString("\n\n")
+
+	if m.healthLoading || m.health == nil {
+		msg := "loading…"
+		if !m.healthLoading && m.health == nil {
+			msg = "no health report available"
+		}
+		b.WriteString(mbg(t, "  "+msg, t.P.Faint) + "\n")
+		b.WriteString("\n" + modalHints(t, [][2]string{{"H/esc", "close"}}))
+		return t.Overlay.Width(w).Render(b.String())
+	}
+
+	h := m.health
+	cw := w - 4
+
+	// overall status header
+	overallFg := t.P.OK
+	if h.Overall == "DEGRADED" {
+		overallFg = t.P.Bad
+	}
+	overall := lipgloss.NewStyle().Foreground(overallFg).Background(t.P.BgPanel).Bold(true).Render(h.Overall)
+	b.WriteString(clip(mbg(t, "  overall  ", t.P.Faint)+overall+mbg(t, "    checked "+h.CheckedAt, t.P.Faint), cw) + "\n\n")
+
+	if len(h.Subjects) == 0 {
+		b.WriteString(mbg(t, "  no health probes recorded — health monitoring may be disabled", t.P.Faint) + "\n")
+		b.WriteString("\n" + modalHints(t, [][2]string{{"H/esc", "close"}}))
+		return t.Overlay.Width(w).Render(b.String())
+	}
+
+	section := func(label string) string {
+		return lipgloss.NewStyle().Foreground(t.P.Accent2).Background(t.P.BgPanel).Bold(true).Render(label)
+	}
+	put := func(styled string) { b.WriteString(clip(styled, cw) + "\n") }
+	faint := func(s string) string { return mbg(t, s, t.P.Faint) }
+
+	for _, sub := range h.Subjects {
+		statusFg := t.P.OK
+		if sub.Status == "DEGRADED" {
+			statusFg = t.P.Bad
+		}
+		statusLabel := lipgloss.NewStyle().Foreground(statusFg).Background(t.P.BgPanel).Bold(sub.Status == "DEGRADED").Render(sub.Status)
+		b.WriteString(section(sub.Subject) + mbg(t, "  ", t.P.BgPanel) + statusLabel + "\n")
+		put(faint(fmt.Sprintf("    metric %-18s value %d", sub.Metric, sub.ObservedValue)))
+		if sub.Detail != "" {
+			put(faint("    " + truncate(sub.Detail, cw-4)))
+		}
+		put(faint("    since  " + sub.Since))
+		b.WriteString("\n")
+	}
+
+	b.WriteString(modalHints(t, [][2]string{{"H/esc", "close"}}))
+	return t.Overlay.Width(w).Render(b.String())
+}
+
 // shortID trims a long artifact id for the list column while staying identifiable.
 func shortID(id string) string {
 	if len(id) <= 14 {