Feat v0.6.4 admin health/metrics tab + cluster merge

Admin Health tab under Monitoring with auto-refreshing runtime, DB,
cluster, and extension metrics panels. New GET /api/v1/admin/metrics
endpoint. Fattened heartbeat JSONB with stack, GC CPU%, extension
count, sandbox stats, trigger fires. Sandbox runner and trigger engine
now track cumulative execution counters. Event bus tracks publish/deliver
counts. Health endpoints consolidated via shared builder. Block renderers
(mermaid, katex, csv-table, diff-viewer) no longer require chat.
cluster-dashboard package retired — merged into admin Health tab.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/cluster/registry_test.go

@@ -75,6 +75,46 @@ func TestCollectStats(t *testing.T) {
 	}
 }
 
+func TestCollectStatsFattened(t *testing.T) {
+	hub := &mockHub{count: 3}
+	reg := NewRegistry("test-node", "", nil, hub, RegistryConfig{
+		HeartbeatInterval: 10 * time.Second,
+		StaleThreshold:    30 * time.Second,
+	})
+	reg.SetSandboxStats(func() (uint64, uint64, float64) { return 100, 5, 12.3 })
+	reg.SetTriggerFireCount(func() int64 { return 42 })
+	reg.SetExtensionCount(func() int { return 8 })
+
+	data := reg.collectStats()
+
+	var stats map[string]any
+	if err := json.Unmarshal(data, &stats); err != nil {
+		t.Fatalf("collectStats returned invalid JSON: %v", err)
+	}
+
+	// New fattened keys
+	fatKeys := []string{
+		"stack_in_use", "gc_cpu_pct",
+		"extensions_loaded", "starlark_exec_total", "starlark_errors_total",
+		"starlark_avg_duration_ms", "trigger_fires_total",
+	}
+	for _, key := range fatKeys {
+		if _, ok := stats[key]; !ok {
+			t.Errorf("missing fattened stats key: %s", key)
+		}
+	}
+
+	if v := stats["extensions_loaded"].(float64); int(v) != 8 {
+		t.Errorf("extensions_loaded = %v, want 8", v)
+	}
+	if v := stats["starlark_exec_total"].(float64); int(v) != 100 {
+		t.Errorf("starlark_exec_total = %v, want 100", v)
+	}
+	if v := stats["trigger_fires_total"].(float64); int(v) != 42 {
+		t.Errorf("trigger_fires_total = %v, want 42", v)
+	}
+}
+
 func TestRegistryStartStop(t *testing.T) {
 	ms := &mockClusterStore{heartbeatRows: 1}
 	hub := &mockHub{count: 0}