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/protocol/golden_test.go

@@ -164,6 +164,39 @@ func TestDecodeGoldenServerFrames(t *testing.T) {
 				}
 			},
 		},
+		{
+			name: "health.checks carries nested health report",
+			json: `{"type":"health.checks","health":{"overall":"DEGRADED","subjects":[` +
+				`{"subject":"DISK","status":"DEGRADED","metric":"free_bytes","observedValue":512000000,` +
+				`"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"}],` +
+				`"checkedAt":"2026-06-14T10:05:00Z"}}`,
+			wantType:  TypeHealthChecks,
+			eventBear: false,
+			check: func(t *testing.T, m ServerMessage) {
+				if m.Health == nil {
+					t.Fatalf("health.checks: Health is nil")
+				}
+				if m.Health.Overall != "DEGRADED" {
+					t.Fatalf("health.checks overall = %q, want DEGRADED", m.Health.Overall)
+				}
+				if len(m.Health.Subjects) != 2 {
+					t.Fatalf("health.checks subjects len = %d, want 2", len(m.Health.Subjects))
+				}
+				disk := m.Health.Subjects[0]
+				if disk.Subject != "DISK" || disk.Status != "DEGRADED" || disk.ObservedValue != 512000000 {
+					t.Fatalf("health.checks DISK subject: %+v", disk)
+				}
+				es := m.Health.Subjects[1]
+				if es.Subject != "EVENT_STORE" || es.Status != "HEALTHY" {
+					t.Fatalf("health.checks EVENT_STORE subject: %+v", es)
+				}
+				if m.Health.CheckedAt != "2026-06-14T10:05:00Z" {
+					t.Fatalf("health.checks checkedAt = %q", m.Health.CheckedAt)
+				}
+			},
+		},
 		{
 			name: "workflow.proposed carries candidate workflows",
 			json: `{"type":"workflow.proposed","sessionId":"s1","proposalId":"prop-1","prompt":"Run one?",` +
@@ -248,6 +281,11 @@ func TestEncodeGoldenClientFrames(t *testing.T) {
 				}
 			},
 		},
+		{
+			name:     "GetHealthChecks encodes correctly",
+			frame:    GetHealthChecks(),
+			wantType: clientPrefix + "GetHealthChecks",
+		},
 		{
 			name:     "DiscardIdea carries the ideaId",
 			frame:    DiscardIdea("i1"),