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/model.go

@@ -53,6 +53,7 @@ const (
 	OverlayConfig
 	OverlayStats
 	OverlayIdeas
+	OverlayHealth
 )
 
 // RouterEntry is one line in a session's conversation transcript.
@@ -259,6 +260,10 @@ type Model struct {
 	ideasIndex   int
 	ideasLoading bool
 
+	// health checks (OverlayHealth) — system-scoped, populated by the health.checks reply
+	health        *protocol.HealthDto
+	healthLoading bool
+
 	// command palette
 	paletteFilter string
 	paletteIndex  int